[V9,1/3] irq: Allow to pass the IRQF_TIMER flag with percpu irq request

Message ID 1493042494-14057-1-git-send-email-daniel.lezcano@linaro.org
State New
Headers show
Series
  • [V9,1/3] irq: Allow to pass the IRQF_TIMER flag with percpu irq request
Related show

Commit Message

Daniel Lezcano April 24, 2017, 2:01 p.m.
In the next changes, we track when the interrupts occur in order to
statistically compute when is supposed to happen the next interrupt.

In all the interruptions, it does not make sense to store the timer interrupt
occurences and try to predict the next interrupt as when know the expiration
time.

The request_irq() has a irq flags parameter and the timer drivers use it to
pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.
Based on this flag, we can discard these interrupts when tracking them.

But, the API request_percpu_irq does not allow to pass a flag, hence specifying
if the interrupt type is a timer.

Add a function request_percpu_irq_flags() where we can specify the flags. The
request_percpu_irq() function is changed to be a wrapper to
request_percpu_irq_flags() passing a zero flag parameter.

Change the timers using request_percpu_irq() to use request_percpu_irq_flags()
instead with the IRQF_TIMER flag set.

For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER
flag (or zero) is a valid parameter to be passed to the
request_percpu_irq_flags() function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Kukjin Kim <kgene@kernel.org>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Javier Martinez Canillas <javier@osg.samsung.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>

---
Changelog:

   V9:
	- Clarified the patch description
	- Fixed EXPORT_SYMBOL_GPL(request_percpu_irq_flags)
---
 arch/arm/kernel/smp_twd.c                |  3 ++-
 drivers/clocksource/arc_timer.c          |  4 ++--
 drivers/clocksource/arm_arch_timer.c     | 20 ++++++++++++--------
 drivers/clocksource/arm_global_timer.c   |  4 ++--
 drivers/clocksource/exynos_mct.c         |  7 ++++---
 drivers/clocksource/qcom-timer.c         |  4 ++--
 drivers/clocksource/time-armada-370-xp.c |  9 +++++----
 drivers/clocksource/timer-nps.c          |  6 +++---
 include/linux/interrupt.h                | 11 ++++++++++-
 kernel/irq/manage.c                      | 15 ++++++++++-----
 virt/kvm/arm/arch_timer.c                |  5 +++--
 11 files changed, 55 insertions(+), 33 deletions(-)

-- 
1.9.1

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Comments

Krzysztof Kozlowski April 24, 2017, 6:33 p.m. | #1
On Mon, Apr 24, 2017 at 04:01:31PM +0200, Daniel Lezcano wrote:
> In the next changes, we track when the interrupts occur in order to

> statistically compute when is supposed to happen the next interrupt.

> 

> In all the interruptions, it does not make sense to store the timer interrupt

> occurences and try to predict the next interrupt as when know the expiration

> time.

> 

> The request_irq() has a irq flags parameter and the timer drivers use it to

> pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.

> Based on this flag, we can discard these interrupts when tracking them.

> 

> But, the API request_percpu_irq does not allow to pass a flag, hence specifying

> if the interrupt type is a timer.

> 

> Add a function request_percpu_irq_flags() where we can specify the flags. The

> request_percpu_irq() function is changed to be a wrapper to

> request_percpu_irq_flags() passing a zero flag parameter.

> 

> Change the timers using request_percpu_irq() to use request_percpu_irq_flags()

> instead with the IRQF_TIMER flag set.

> 

> For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER

> flag (or zero) is a valid parameter to be passed to the

> request_percpu_irq_flags() function.

> 

> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

> Cc: Mark Rutland <mark.rutland@arm.com>

> Cc: Vineet Gupta <vgupta@synopsys.com>

> Cc: Marc Zyngier <marc.zyngier@arm.com>

> Cc: Patrice Chotard <patrice.chotard@st.com>

> Cc: Kukjin Kim <kgene@kernel.org>

> Cc: Krzysztof Kozlowski <krzk@kernel.org>

> Cc: Javier Martinez Canillas <javier@osg.samsung.com>

> Cc: Christoffer Dall <christoffer.dall@linaro.org>

> Cc: Paolo Bonzini <pbonzini@redhat.com>

> Cc: Radim Krčmář <rkrcmar@redhat.com>

> 

> ---

> Changelog:

> 

>    V9:

> 	- Clarified the patch description

> 	- Fixed EXPORT_SYMBOL_GPL(request_percpu_irq_flags)

> ---

>  arch/arm/kernel/smp_twd.c                |  3 ++-

>  drivers/clocksource/arc_timer.c          |  4 ++--

>  drivers/clocksource/arm_arch_timer.c     | 20 ++++++++++++--------

>  drivers/clocksource/arm_global_timer.c   |  4 ++--

>  drivers/clocksource/exynos_mct.c         |  7 ++++---

>  drivers/clocksource/qcom-timer.c         |  4 ++--

>  drivers/clocksource/time-armada-370-xp.c |  9 +++++----

>  drivers/clocksource/timer-nps.c          |  6 +++---

>  include/linux/interrupt.h                | 11 ++++++++++-

>  kernel/irq/manage.c                      | 15 ++++++++++-----

>  virt/kvm/arm/arch_timer.c                |  5 +++--

>  11 files changed, 55 insertions(+), 33 deletions(-)

>


For exynos-mct:
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>


Best regards,
Krzysztof

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Marc Zyngier April 24, 2017, 6:46 p.m. | #2
On 24/04/17 15:01, Daniel Lezcano wrote:
> In the next changes, we track when the interrupts occur in order to

> statistically compute when is supposed to happen the next interrupt.

> 

> In all the interruptions, it does not make sense to store the timer interrupt

> occurences and try to predict the next interrupt as when know the expiration

> time.

> 

> The request_irq() has a irq flags parameter and the timer drivers use it to

> pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.

> Based on this flag, we can discard these interrupts when tracking them.

> 

> But, the API request_percpu_irq does not allow to pass a flag, hence specifying

> if the interrupt type is a timer.

> 

> Add a function request_percpu_irq_flags() where we can specify the flags. The

> request_percpu_irq() function is changed to be a wrapper to

> request_percpu_irq_flags() passing a zero flag parameter.

> 

> Change the timers using request_percpu_irq() to use request_percpu_irq_flags()

> instead with the IRQF_TIMER flag set.

> 

> For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER

> flag (or zero) is a valid parameter to be passed to the

> request_percpu_irq_flags() function.


[...]

> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c

> index 35d7100..602e0a8 100644

> --- a/virt/kvm/arm/arch_timer.c

> +++ b/virt/kvm/arm/arch_timer.c

> @@ -523,8 +523,9 @@ int kvm_timer_hyp_init(void)

>  		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;

>  	}

>  

> -	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,

> -				 "kvm guest timer", kvm_get_running_vcpus());

> +	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,

> +				       IRQF_TIMER, "kvm guest timer",

> +				       kvm_get_running_vcpus());

>  	if (err) {

>  		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",

>  			host_vtimer_irq, err);

> 


How is that useful? This timer is controlled by the guest OS, and not
the host kernel. Can you explain how you intend to make use of that
information in this case?

Thanks,

	M.
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Daniel Lezcano April 24, 2017, 6:59 p.m. | #3
On Mon, Apr 24, 2017 at 07:46:43PM +0100, Marc Zyngier wrote:
> On 24/04/17 15:01, Daniel Lezcano wrote:

> > In the next changes, we track when the interrupts occur in order to

> > statistically compute when is supposed to happen the next interrupt.

> > 

> > In all the interruptions, it does not make sense to store the timer interrupt

> > occurences and try to predict the next interrupt as when know the expiration

> > time.

> > 

> > The request_irq() has a irq flags parameter and the timer drivers use it to

> > pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.

> > Based on this flag, we can discard these interrupts when tracking them.

> > 

> > But, the API request_percpu_irq does not allow to pass a flag, hence specifying

> > if the interrupt type is a timer.

> > 

> > Add a function request_percpu_irq_flags() where we can specify the flags. The

> > request_percpu_irq() function is changed to be a wrapper to

> > request_percpu_irq_flags() passing a zero flag parameter.

> > 

> > Change the timers using request_percpu_irq() to use request_percpu_irq_flags()

> > instead with the IRQF_TIMER flag set.

> > 

> > For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER

> > flag (or zero) is a valid parameter to be passed to the

> > request_percpu_irq_flags() function.

> 

> [...]

> 

> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c

> > index 35d7100..602e0a8 100644

> > --- a/virt/kvm/arm/arch_timer.c

> > +++ b/virt/kvm/arm/arch_timer.c

> > @@ -523,8 +523,9 @@ int kvm_timer_hyp_init(void)

> >  		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;

> >  	}

> >  

> > -	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,

> > -				 "kvm guest timer", kvm_get_running_vcpus());

> > +	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,

> > +				       IRQF_TIMER, "kvm guest timer",

> > +				       kvm_get_running_vcpus());

> >  	if (err) {

> >  		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",

> >  			host_vtimer_irq, err);

> > 

> 

> How is that useful? This timer is controlled by the guest OS, and not

> the host kernel. Can you explain how you intend to make use of that

> information in this case?


Isn't it a source of interruption on the host kernel?

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Marc Zyngier April 24, 2017, 7:14 p.m. | #4
On 24/04/17 19:59, Daniel Lezcano wrote:
> On Mon, Apr 24, 2017 at 07:46:43PM +0100, Marc Zyngier wrote:

>> On 24/04/17 15:01, Daniel Lezcano wrote:

>>> In the next changes, we track when the interrupts occur in order to

>>> statistically compute when is supposed to happen the next interrupt.

>>>

>>> In all the interruptions, it does not make sense to store the timer interrupt

>>> occurences and try to predict the next interrupt as when know the expiration

>>> time.

>>>

>>> The request_irq() has a irq flags parameter and the timer drivers use it to

>>> pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.

>>> Based on this flag, we can discard these interrupts when tracking them.

>>>

>>> But, the API request_percpu_irq does not allow to pass a flag, hence specifying

>>> if the interrupt type is a timer.

>>>

>>> Add a function request_percpu_irq_flags() where we can specify the flags. The

>>> request_percpu_irq() function is changed to be a wrapper to

>>> request_percpu_irq_flags() passing a zero flag parameter.

>>>

>>> Change the timers using request_percpu_irq() to use request_percpu_irq_flags()

>>> instead with the IRQF_TIMER flag set.

>>>

>>> For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER

>>> flag (or zero) is a valid parameter to be passed to the

>>> request_percpu_irq_flags() function.

>>

>> [...]

>>

>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c

>>> index 35d7100..602e0a8 100644

>>> --- a/virt/kvm/arm/arch_timer.c

>>> +++ b/virt/kvm/arm/arch_timer.c

>>> @@ -523,8 +523,9 @@ int kvm_timer_hyp_init(void)

>>>  		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;

>>>  	}

>>>  

>>> -	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,

>>> -				 "kvm guest timer", kvm_get_running_vcpus());

>>> +	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,

>>> +				       IRQF_TIMER, "kvm guest timer",

>>> +				       kvm_get_running_vcpus());

>>>  	if (err) {

>>>  		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",

>>>  			host_vtimer_irq, err);

>>>

>>

>> How is that useful? This timer is controlled by the guest OS, and not

>> the host kernel. Can you explain how you intend to make use of that

>> information in this case?

> 

> Isn't it a source of interruption on the host kernel?


Only to cause an exit of the VM, and not under the control of the host.
This isn't triggering any timer related action on the host code either.

Your patch series seems to assume some kind of predictability of the
timer interrupt, which can make sense on the host. Here, this interrupt
is shared among *all* guests running on this system.

Maybe you could explain why you think this interrupt is relevant to what
you're trying to achieve?

Thanks,

	M.
-- 
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Daniel Lezcano April 25, 2017, 8:34 a.m. | #5
On Tue, Apr 25, 2017 at 08:38:56AM +0100, Marc Zyngier wrote:
> On 24/04/17 20:59, Daniel Lezcano wrote:

> > On Mon, Apr 24, 2017 at 08:14:54PM +0100, Marc Zyngier wrote:

> >> On 24/04/17 19:59, Daniel Lezcano wrote:

> >>> On Mon, Apr 24, 2017 at 07:46:43PM +0100, Marc Zyngier wrote:

> >>>> On 24/04/17 15:01, Daniel Lezcano wrote:

> >>>>> In the next changes, we track when the interrupts occur in order to

> >>>>> statistically compute when is supposed to happen the next interrupt.

> >>>>>

> >>>>> In all the interruptions, it does not make sense to store the timer interrupt

> >>>>> occurences and try to predict the next interrupt as when know the expiration

> >>>>> time.

> >>>>>

> >>>>> The request_irq() has a irq flags parameter and the timer drivers use it to

> >>>>> pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.

> >>>>> Based on this flag, we can discard these interrupts when tracking them.

> >>>>>

> >>>>> But, the API request_percpu_irq does not allow to pass a flag, hence specifying

> >>>>> if the interrupt type is a timer.

> >>>>>

> >>>>> Add a function request_percpu_irq_flags() where we can specify the flags. The

> >>>>> request_percpu_irq() function is changed to be a wrapper to

> >>>>> request_percpu_irq_flags() passing a zero flag parameter.

> >>>>>

> >>>>> Change the timers using request_percpu_irq() to use request_percpu_irq_flags()

> >>>>> instead with the IRQF_TIMER flag set.

> >>>>>

> >>>>> For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER

> >>>>> flag (or zero) is a valid parameter to be passed to the

> >>>>> request_percpu_irq_flags() function.

> >>>>

> >>>> [...]

> >>>>

> >>>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c

> >>>>> index 35d7100..602e0a8 100644

> >>>>> --- a/virt/kvm/arm/arch_timer.c

> >>>>> +++ b/virt/kvm/arm/arch_timer.c

> >>>>> @@ -523,8 +523,9 @@ int kvm_timer_hyp_init(void)

> >>>>>  		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;

> >>>>>  	}

> >>>>>  

> >>>>> -	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,

> >>>>> -				 "kvm guest timer", kvm_get_running_vcpus());

> >>>>> +	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,

> >>>>> +				       IRQF_TIMER, "kvm guest timer",

> >>>>> +				       kvm_get_running_vcpus());

> >>>>>  	if (err) {

> >>>>>  		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",

> >>>>>  			host_vtimer_irq, err);

> >>>>>

> >>>>

> >>>> How is that useful? This timer is controlled by the guest OS, and not

> >>>> the host kernel. Can you explain how you intend to make use of that

> >>>> information in this case?

> >>>

> >>> Isn't it a source of interruption on the host kernel?

> >>

> >> Only to cause an exit of the VM, and not under the control of the host.

> >> This isn't triggering any timer related action on the host code either.

> >>

> >> Your patch series seems to assume some kind of predictability of the

> >> timer interrupt, which can make sense on the host. Here, this interrupt

> >> is shared among *all* guests running on this system.

> >>

> >> Maybe you could explain why you think this interrupt is relevant to what

> >> you're trying to achieve?

> > 

> > If this interrupt does not happen on the host, we don't care.

> 

> All interrupts happen on the host. There is no such thing as a HW 

> interrupt being directly delivered to a guest (at least so far). The 

> timer is under control of the guest, which uses as it sees fit. When 

> the HW timer expires, the interrupt fires on the host, which re-inject 

> the interrupt in the guest.


Ah, thanks for the clarification. Interesting.

How can the host know which guest to re-inject the interrupt?

> > The flag IRQF_TIMER is used by the spurious irq handler in the try_one_irq()

> > function. However the per cpu timer interrupt will be discarded in the function

> > before because it is per cpu.

> 

> Right. That's not because this is a timer, but because it is per-cpu. 

> So why do we need this IRQF_TIMER flag, instead of fixing try_one_irq()?


When a timer is not per cpu, (eg. request_irq), we need this flag, no?

> > IMO, for consistency reason, adding the IRQF_TIMER makes sense. Other than

> > that, as the interrupt is not happening on the host, this flag won't be used.

> > 

> > Do you want to drop this change?

> 

> No, I'd like to understand the above. Why isn't the following patch 

> doing the right thing?


Actually, the explanation is in the next patch of the series (2/3)

[ ... ]

+static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)
+{
+	/*
+	 * We don't need the measurement because the idle code already
+	 * knows the next expiry event.
+	 */
+	if (act->flags & __IRQF_TIMER)
+		return;
+
+	desc->istate |= IRQS_TIMINGS;
+}

[ ... ]

+/*
+ * The function record_irq_time is only called in one place in the
+ * interrupts handler. We want this function always inline so the code
+ * inside is embedded in the function and the static key branching
+ * code can act at the higher level. Without the explicit
+ * __always_inline we can end up with a function call and a small
+ * overhead in the hotpath for nothing.
+ */
+static __always_inline void record_irq_time(struct irq_desc *desc)
+{
+	if (!static_branch_likely(&irq_timing_enabled))
+		return;
+
+	if (desc->istate & IRQS_TIMINGS) {
+		struct irq_timings *timings = this_cpu_ptr(&irq_timings);
+
+		timings->values[timings->count & IRQ_TIMINGS_MASK] =
+			irq_timing_encode(local_clock(),
+					  irq_desc_get_irq(desc));
+
+		timings->count++;
+	}
+}

[ ... ]

The purpose is to predict the next event interrupts on the system which are
source of wake up. For now, this patchset is focused on interrupts (discarding
timer interrupts).

The following article gives more details: https://lwn.net/Articles/673641/

When the interrupt is setup, we tag it except if it is a timer. So with this
patch there is another usage of the IRQF_TIMER where we will be ignoring
interrupt coming from a timer.

As the timer interrupt is delivered to the host, we should not measure it as it
is a timer and set this flag.

The needed information is: "what is the earliest VM timer?". If this
information is already available then there is nothing more to do, otherwise we
should add it in the future.

> diff --git a/kernel/irq/spurious.c b/kernel/irq/spurious.c

> index 061ba7eed4ed..a4a81c6c7602 100644

> --- a/kernel/irq/spurious.c

> +++ b/kernel/irq/spurious.c

> @@ -72,6 +72,7 @@ static int try_one_irq(struct irq_desc *desc, bool force)

>  	 * marked polled are excluded from polling.

>  	 */

>  	if (irq_settings_is_per_cpu(desc) ||

> +	    irq_settings_is_per_cpu_devid(desc) ||

>  	    irq_settings_is_nested_thread(desc) ||

>  	    irq_settings_is_polled(desc))

>  		goto out;

> 

> Thanks,

> 

> 	M.

> -- 

> Jazz is not dead. It just smells funny...


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Marc Zyngier April 25, 2017, 9:10 a.m. | #6
On 25/04/17 09:34, Daniel Lezcano wrote:
> On Tue, Apr 25, 2017 at 08:38:56AM +0100, Marc Zyngier wrote:

>> On 24/04/17 20:59, Daniel Lezcano wrote:

>>> On Mon, Apr 24, 2017 at 08:14:54PM +0100, Marc Zyngier wrote:

>>>> On 24/04/17 19:59, Daniel Lezcano wrote:

>>>>> On Mon, Apr 24, 2017 at 07:46:43PM +0100, Marc Zyngier wrote:

>>>>>> On 24/04/17 15:01, Daniel Lezcano wrote:

>>>>>>> In the next changes, we track when the interrupts occur in order to

>>>>>>> statistically compute when is supposed to happen the next interrupt.

>>>>>>>

>>>>>>> In all the interruptions, it does not make sense to store the timer interrupt

>>>>>>> occurences and try to predict the next interrupt as when know the expiration

>>>>>>> time.

>>>>>>>

>>>>>>> The request_irq() has a irq flags parameter and the timer drivers use it to

>>>>>>> pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.

>>>>>>> Based on this flag, we can discard these interrupts when tracking them.

>>>>>>>

>>>>>>> But, the API request_percpu_irq does not allow to pass a flag, hence specifying

>>>>>>> if the interrupt type is a timer.

>>>>>>>

>>>>>>> Add a function request_percpu_irq_flags() where we can specify the flags. The

>>>>>>> request_percpu_irq() function is changed to be a wrapper to

>>>>>>> request_percpu_irq_flags() passing a zero flag parameter.

>>>>>>>

>>>>>>> Change the timers using request_percpu_irq() to use request_percpu_irq_flags()

>>>>>>> instead with the IRQF_TIMER flag set.

>>>>>>>

>>>>>>> For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER

>>>>>>> flag (or zero) is a valid parameter to be passed to the

>>>>>>> request_percpu_irq_flags() function.

>>>>>>

>>>>>> [...]

>>>>>>

>>>>>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c

>>>>>>> index 35d7100..602e0a8 100644

>>>>>>> --- a/virt/kvm/arm/arch_timer.c

>>>>>>> +++ b/virt/kvm/arm/arch_timer.c

>>>>>>> @@ -523,8 +523,9 @@ int kvm_timer_hyp_init(void)

>>>>>>>  		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;

>>>>>>>  	}

>>>>>>>  

>>>>>>> -	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,

>>>>>>> -				 "kvm guest timer", kvm_get_running_vcpus());

>>>>>>> +	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,

>>>>>>> +				       IRQF_TIMER, "kvm guest timer",

>>>>>>> +				       kvm_get_running_vcpus());

>>>>>>>  	if (err) {

>>>>>>>  		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",

>>>>>>>  			host_vtimer_irq, err);

>>>>>>>

>>>>>>

>>>>>> How is that useful? This timer is controlled by the guest OS, and not

>>>>>> the host kernel. Can you explain how you intend to make use of that

>>>>>> information in this case?

>>>>>

>>>>> Isn't it a source of interruption on the host kernel?

>>>>

>>>> Only to cause an exit of the VM, and not under the control of the host.

>>>> This isn't triggering any timer related action on the host code either.

>>>>

>>>> Your patch series seems to assume some kind of predictability of the

>>>> timer interrupt, which can make sense on the host. Here, this interrupt

>>>> is shared among *all* guests running on this system.

>>>>

>>>> Maybe you could explain why you think this interrupt is relevant to what

>>>> you're trying to achieve?

>>>

>>> If this interrupt does not happen on the host, we don't care.

>>

>> All interrupts happen on the host. There is no such thing as a HW 

>> interrupt being directly delivered to a guest (at least so far). The 

>> timer is under control of the guest, which uses as it sees fit. When 

>> the HW timer expires, the interrupt fires on the host, which re-inject 

>> the interrupt in the guest.

> 

> Ah, thanks for the clarification. Interesting.

> 

> How can the host know which guest to re-inject the interrupt?


The timer can only fire when the vcpu is running. If it is not running,
a software timer is queued, with a pointer to the vcpu struct.

>>> The flag IRQF_TIMER is used by the spurious irq handler in the try_one_irq()

>>> function. However the per cpu timer interrupt will be discarded in the function

>>> before because it is per cpu.

>>

>> Right. That's not because this is a timer, but because it is per-cpu. 

>> So why do we need this IRQF_TIMER flag, instead of fixing try_one_irq()?

> 

> When a timer is not per cpu, (eg. request_irq), we need this flag, no?


Sure, but in this series, they all seem to be per-cpu.

>>> IMO, for consistency reason, adding the IRQF_TIMER makes sense. Other than

>>> that, as the interrupt is not happening on the host, this flag won't be used.

>>>

>>> Do you want to drop this change?

>>

>> No, I'd like to understand the above. Why isn't the following patch 

>> doing the right thing?

> 

> Actually, the explanation is in the next patch of the series (2/3)

> 

> [ ... ]

> 

> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)

> +{

> +	/*

> +	 * We don't need the measurement because the idle code already

> +	 * knows the next expiry event.

> +	 */

> +	if (act->flags & __IRQF_TIMER)

> +		return;


And that's where this is really wrong for the KVM guest timer. As I
said, this timer is under complete control of the guest, and the rest of
the system doesn't know about it. KVM itself will only find out when the
vcpu does a VM exit for a reason or another, and will just save/restore
the state in order to be able to give the timer to another guest.

The idle code is very much *not* aware of anything concerning that guest
timer.

> +

> +	desc->istate |= IRQS_TIMINGS;

> +}

> 

> [ ... ]

> 

> +/*

> + * The function record_irq_time is only called in one place in the

> + * interrupts handler. We want this function always inline so the code

> + * inside is embedded in the function and the static key branching

> + * code can act at the higher level. Without the explicit

> + * __always_inline we can end up with a function call and a small

> + * overhead in the hotpath for nothing.

> + */

> +static __always_inline void record_irq_time(struct irq_desc *desc)

> +{

> +	if (!static_branch_likely(&irq_timing_enabled))

> +		return;

> +

> +	if (desc->istate & IRQS_TIMINGS) {

> +		struct irq_timings *timings = this_cpu_ptr(&irq_timings);

> +

> +		timings->values[timings->count & IRQ_TIMINGS_MASK] =

> +			irq_timing_encode(local_clock(),

> +					  irq_desc_get_irq(desc));

> +

> +		timings->count++;

> +	}

> +}

> 

> [ ... ]

> 

> The purpose is to predict the next event interrupts on the system which are

> source of wake up. For now, this patchset is focused on interrupts (discarding

> timer interrupts).

> 

> The following article gives more details: https://lwn.net/Articles/673641/

> 

> When the interrupt is setup, we tag it except if it is a timer. So with this

> patch there is another usage of the IRQF_TIMER where we will be ignoring

> interrupt coming from a timer.

> 

> As the timer interrupt is delivered to the host, we should not measure it as it

> is a timer and set this flag.

> 

> The needed information is: "what is the earliest VM timer?". If this

> information is already available then there is nothing more to do, otherwise we

> should add it in the future.


This information is not readily available. You can only find it when it
is too late (timer has already fired) or when it is not relevant anymore
(guest is sleeping and we've queued a SW timer for it).

Thanks,

	M.
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Marc Zyngier April 25, 2017, 10:21 a.m. | #7
On 25/04/17 10:49, Daniel Lezcano wrote:
> On Tue, Apr 25, 2017 at 10:10:12AM +0100, Marc Zyngier wrote:


[...]

>>> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)

>>> +{

>>> +	/*

>>> +	 * We don't need the measurement because the idle code already

>>> +	 * knows the next expiry event.

>>> +	 */

>>> +	if (act->flags & __IRQF_TIMER)

>>> +		return;

>>

>> And that's where this is really wrong for the KVM guest timer. As I

>> said, this timer is under complete control of the guest, and the rest of

>> the system doesn't know about it. KVM itself will only find out when the

>> vcpu does a VM exit for a reason or another, and will just save/restore

>> the state in order to be able to give the timer to another guest.

>>

>> The idle code is very much *not* aware of anything concerning that guest

>> timer.

> 

> Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1

> at <time> and exits, VM2 runs and sets a timer2 at <time+delta>.

> 

> The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual

> timer is under control of VM2 and will expire at <time+delta>.

> 

> Is the host wake up with the SW timer and switch in VM1 which in turn restores

> the timer and jump in the virtual timer irq handler?


Indeed. The SW timer causes VM1 to wake-up, either on the same CPU
(preempting VM2) or on another. The timer is then restored with the
pending virtual interrupt injected, and the guest does what it has to
with it.

Thanks,

	M.
-- 
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Daniel Lezcano April 25, 2017, 12:51 p.m. | #8
On Tue, Apr 25, 2017 at 11:21:21AM +0100, Marc Zyngier wrote:
> On 25/04/17 10:49, Daniel Lezcano wrote:

> > On Tue, Apr 25, 2017 at 10:10:12AM +0100, Marc Zyngier wrote:

> 

> [...]

> 

> >>> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)

> >>> +{

> >>> +	/*

> >>> +	 * We don't need the measurement because the idle code already

> >>> +	 * knows the next expiry event.

> >>> +	 */

> >>> +	if (act->flags & __IRQF_TIMER)

> >>> +		return;

> >>

> >> And that's where this is really wrong for the KVM guest timer. As I

> >> said, this timer is under complete control of the guest, and the rest of

> >> the system doesn't know about it. KVM itself will only find out when the

> >> vcpu does a VM exit for a reason or another, and will just save/restore

> >> the state in order to be able to give the timer to another guest.

> >>

> >> The idle code is very much *not* aware of anything concerning that guest

> >> timer.

> > 

> > Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1

> > at <time> and exits, VM2 runs and sets a timer2 at <time+delta>.

> > 

> > The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual

> > timer is under control of VM2 and will expire at <time+delta>.

> > 

> > Is the host wake up with the SW timer and switch in VM1 which in turn restores

> > the timer and jump in the virtual timer irq handler?

> 

> Indeed. The SW timer causes VM1 to wake-up, either on the same CPU

> (preempting VM2) or on another. The timer is then restored with the

> pending virtual interrupt injected, and the guest does what it has to

> with it.


Thanks for clarification.

So there is a virtual timer with real registers / interruption (waking up the
host) for the running VMs and SW timers for non-running VMs.

What is the benefit of having such mechanism instead of real timers injecting
interrupts in the VM without the virtual timer + save/restore? Efficiency in
the running VMs when setting up timers (saving privileges change overhead)?

-- 

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Daniel Lezcano April 25, 2017, 1:53 p.m. | #9
On 25/04/2017 15:22, Marc Zyngier wrote:
> On 25/04/17 13:51, Daniel Lezcano wrote:

>> On Tue, Apr 25, 2017 at 11:21:21AM +0100, Marc Zyngier wrote:

>>> On 25/04/17 10:49, Daniel Lezcano wrote:

>>>> On Tue, Apr 25, 2017 at 10:10:12AM +0100, Marc Zyngier wrote:

>>>

>>> [...]

>>>

>>>>>> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)

>>>>>> +{

>>>>>> +	/*

>>>>>> +	 * We don't need the measurement because the idle code already

>>>>>> +	 * knows the next expiry event.

>>>>>> +	 */

>>>>>> +	if (act->flags & __IRQF_TIMER)

>>>>>> +		return;

>>>>>

>>>>> And that's where this is really wrong for the KVM guest timer. As I

>>>>> said, this timer is under complete control of the guest, and the rest of

>>>>> the system doesn't know about it. KVM itself will only find out when the

>>>>> vcpu does a VM exit for a reason or another, and will just save/restore

>>>>> the state in order to be able to give the timer to another guest.

>>>>>

>>>>> The idle code is very much *not* aware of anything concerning that guest

>>>>> timer.

>>>>

>>>> Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1

>>>> at <time> and exits, VM2 runs and sets a timer2 at <time+delta>.

>>>>

>>>> The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual

>>>> timer is under control of VM2 and will expire at <time+delta>.

>>>>

>>>> Is the host wake up with the SW timer and switch in VM1 which in turn restores

>>>> the timer and jump in the virtual timer irq handler?

>>>

>>> Indeed. The SW timer causes VM1 to wake-up, either on the same CPU

>>> (preempting VM2) or on another. The timer is then restored with the

>>> pending virtual interrupt injected, and the guest does what it has to

>>> with it.

>>

>> Thanks for clarification.

>>

>> So there is a virtual timer with real registers / interruption (waking up the

>> host) for the running VMs and SW timers for non-running VMs.

>>

>> What is the benefit of having such mechanism instead of real timers injecting

>> interrupts in the VM without the virtual timer + save/restore? Efficiency in

>> the running VMs when setting up timers (saving privileges change overhead)?

> 

> 

> You can't dedicate HW resources to virtual CPUs. It just doesn't scale.

> Also, injecting HW interrupts in a guest is pretty hard work, and for

> multiple reasons:

>   - the host needs to be in control of interrupt delivery (don't hog the

> CPU with guest interrupts)

>   - you want to be able to remap interrupts (id X on the host becomes id

> Y on the guest),

>   - you want to deal with migrating vcpus,

>   - you want deliver an interrupt to a vcpu that is *not* running.

> 

> It *is* doable, but it is not cheap at all from a HW point of view.



Ok, I see.

Thanks!

  -- Daniel


-- 
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Patch hide | download patch | download mbox

diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
index 895ae51..ce9fdcf 100644
--- a/arch/arm/kernel/smp_twd.c
+++ b/arch/arm/kernel/smp_twd.c
@@ -332,7 +332,8 @@  static int __init twd_local_timer_common_register(struct device_node *np)
 		goto out_free;
 	}
 
-	err = request_percpu_irq(twd_ppi, twd_handler, "twd", twd_evt);
+	err = request_percpu_irq_flags(twd_ppi, twd_handler, IRQF_TIMER, "twd",
+				       twd_evt);
 	if (err) {
 		pr_err("twd: can't register interrupt %d (%d)\n", twd_ppi, err);
 		goto out_free;
diff --git a/drivers/clocksource/arc_timer.c b/drivers/clocksource/arc_timer.c
index 7517f95..993e6af 100644
--- a/drivers/clocksource/arc_timer.c
+++ b/drivers/clocksource/arc_timer.c
@@ -301,8 +301,8 @@  static int __init arc_clockevent_setup(struct device_node *node)
 	}
 
 	/* Needs apriori irq_set_percpu_devid() done in intc map function */
-	ret = request_percpu_irq(arc_timer_irq, timer_irq_handler,
-				 "Timer0 (per-cpu-tick)", evt);
+	ret = request_percpu_irq_flags(arc_timer_irq, timer_irq_handler, IRQF_TIMER,
+				       "Timer0 (per-cpu-tick)", evt);
 	if (ret) {
 		pr_err("clockevent: unable to request irq\n");
 		return ret;
diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 7a8a411..d9d00b0 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -768,25 +768,29 @@  static int __init arch_timer_register(void)
 	ppi = arch_timer_ppi[arch_timer_uses_ppi];
 	switch (arch_timer_uses_ppi) {
 	case VIRT_PPI:
-		err = request_percpu_irq(ppi, arch_timer_handler_virt,
-					 "arch_timer", arch_timer_evt);
+		err = request_percpu_irq_flags(ppi, arch_timer_handler_virt,
+					       IRQF_TIMER, "arch_timer",
+					       arch_timer_evt);
 		break;
 	case PHYS_SECURE_PPI:
 	case PHYS_NONSECURE_PPI:
-		err = request_percpu_irq(ppi, arch_timer_handler_phys,
-					 "arch_timer", arch_timer_evt);
+		err = request_percpu_irq_flags(ppi, arch_timer_handler_phys,
+					       IRQF_TIMER, "arch_timer",
+					       arch_timer_evt);
 		if (!err && arch_timer_ppi[PHYS_NONSECURE_PPI]) {
 			ppi = arch_timer_ppi[PHYS_NONSECURE_PPI];
-			err = request_percpu_irq(ppi, arch_timer_handler_phys,
-						 "arch_timer", arch_timer_evt);
+			err = request_percpu_irq_flags(ppi, arch_timer_handler_phys,
+						       IRQF_TIMER, "arch_timer",
+						       arch_timer_evt);
 			if (err)
 				free_percpu_irq(arch_timer_ppi[PHYS_SECURE_PPI],
 						arch_timer_evt);
 		}
 		break;
 	case HYP_PPI:
-		err = request_percpu_irq(ppi, arch_timer_handler_phys,
-					 "arch_timer", arch_timer_evt);
+		err = request_percpu_irq_flags(ppi, arch_timer_handler_phys,
+					       IRQF_TIMER, "arch_timer",
+					       arch_timer_evt);
 		break;
 	default:
 		BUG();
diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
index 123ed20..5a72ec1 100644
--- a/drivers/clocksource/arm_global_timer.c
+++ b/drivers/clocksource/arm_global_timer.c
@@ -302,8 +302,8 @@  static int __init global_timer_of_register(struct device_node *np)
 		goto out_clk;
 	}
 
-	err = request_percpu_irq(gt_ppi, gt_clockevent_interrupt,
-				 "gt", gt_evt);
+	err = request_percpu_irq_flags(gt_ppi, gt_clockevent_interrupt,
+				       IRQF_TIMER, "gt", gt_evt);
 	if (err) {
 		pr_warn("global-timer: can't register interrupt %d (%d)\n",
 			gt_ppi, err);
diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
index 670ff0f..a48ca0f 100644
--- a/drivers/clocksource/exynos_mct.c
+++ b/drivers/clocksource/exynos_mct.c
@@ -524,9 +524,10 @@  static int __init exynos4_timer_resources(struct device_node *np, void __iomem *
 
 	if (mct_int_type == MCT_INT_PPI) {
 
-		err = request_percpu_irq(mct_irqs[MCT_L0_IRQ],
-					 exynos4_mct_tick_isr, "MCT",
-					 &percpu_mct_tick);
+		err = request_percpu_irq_flags(mct_irqs[MCT_L0_IRQ],
+					       exynos4_mct_tick_isr,
+					       IRQF_TIMER, "MCT",
+					       &percpu_mct_tick);
 		WARN(err, "MCT: can't request IRQ %d (%d)\n",
 		     mct_irqs[MCT_L0_IRQ], err);
 	} else {
diff --git a/drivers/clocksource/qcom-timer.c b/drivers/clocksource/qcom-timer.c
index ee358cd..8e876fc 100644
--- a/drivers/clocksource/qcom-timer.c
+++ b/drivers/clocksource/qcom-timer.c
@@ -174,8 +174,8 @@  static int __init msm_timer_init(u32 dgt_hz, int sched_bits, int irq,
 	}
 
 	if (percpu)
-		res = request_percpu_irq(irq, msm_timer_interrupt,
-					 "gp_timer", msm_evt);
+		res = request_percpu_irq_flags(irq, msm_timer_interrupt,
+					       IRQF_TIMER, "gp_timer", msm_evt);
 
 	if (res) {
 		pr_err("request_percpu_irq failed\n");
diff --git a/drivers/clocksource/time-armada-370-xp.c b/drivers/clocksource/time-armada-370-xp.c
index 4440aef..7405e14 100644
--- a/drivers/clocksource/time-armada-370-xp.c
+++ b/drivers/clocksource/time-armada-370-xp.c
@@ -309,10 +309,11 @@  static int __init armada_370_xp_timer_common_init(struct device_node *np)
 	/*
 	 * Setup clockevent timer (interrupt-driven).
 	 */
-	res = request_percpu_irq(armada_370_xp_clkevt_irq,
-				armada_370_xp_timer_interrupt,
-				"armada_370_xp_per_cpu_tick",
-				armada_370_xp_evt);
+	res = request_percpu_irq_flags(armada_370_xp_clkevt_irq,
+				       armada_370_xp_timer_interrupt,
+				       IRQF_TIMER,
+				       "armada_370_xp_per_cpu_tick",
+				       armada_370_xp_evt);
 	/* Immediately configure the timer on the boot CPU */
 	if (res) {
 		pr_err("Failed to request percpu irq");
diff --git a/drivers/clocksource/timer-nps.c b/drivers/clocksource/timer-nps.c
index da1f798..195f039 100644
--- a/drivers/clocksource/timer-nps.c
+++ b/drivers/clocksource/timer-nps.c
@@ -256,9 +256,9 @@  static int __init nps_setup_clockevent(struct device_node *node)
 		return ret;
 
 	/* Needs apriori irq_set_percpu_devid() done in intc map function */
-	ret = request_percpu_irq(nps_timer0_irq, timer_irq_handler,
-				 "Timer0 (per-cpu-tick)",
-				 &nps_clockevent_device);
+	ret = request_percpu_irq_flags(nps_timer0_irq, timer_irq_handler,
+				       IRQF_TIMER, "Timer0 (per-cpu-tick)",
+				       &nps_clockevent_device);
 	if (ret) {
 		pr_err("Couldn't request irq\n");
 		clk_disable_unprepare(clk);
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 53144e7..8f44f23 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -152,8 +152,17 @@  struct irqaction {
 			unsigned long flags, const char *name, void *dev_id);
 
 extern int __must_check
+request_percpu_irq_flags(unsigned int irq, irq_handler_t handler,
+			 unsigned long flags, const char *devname,
+			 void __percpu *percpu_dev_id);
+
+static inline int __must_check
 request_percpu_irq(unsigned int irq, irq_handler_t handler,
-		   const char *devname, void __percpu *percpu_dev_id);
+		   const char *devname, void __percpu *percpu_dev_id)
+{
+	return request_percpu_irq_flags(irq, handler, 0,
+					devname, percpu_dev_id);
+}
 
 extern void free_irq(unsigned int, void *);
 extern void free_percpu_irq(unsigned int, void __percpu *);
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index ae1c90f..1ba7734 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -1951,9 +1951,10 @@  int setup_percpu_irq(unsigned int irq, struct irqaction *act)
 }
 
 /**
- *	request_percpu_irq - allocate a percpu interrupt line
+ *	request_percpu_irq_flags - allocate a percpu interrupt line
  *	@irq: Interrupt line to allocate
  *	@handler: Function to be called when the IRQ occurs.
+ *	@flags: Interrupt type flags (IRQF_TIMER only)
  *	@devname: An ascii name for the claiming device
  *	@dev_id: A percpu cookie passed back to the handler function
  *
@@ -1966,8 +1967,9 @@  int setup_percpu_irq(unsigned int irq, struct irqaction *act)
  *	the handler gets called with the interrupted CPU's instance of
  *	that variable.
  */
-int request_percpu_irq(unsigned int irq, irq_handler_t handler,
-		       const char *devname, void __percpu *dev_id)
+int request_percpu_irq_flags(unsigned int irq, irq_handler_t handler,
+			     unsigned long flags, const char *devname,
+			     void __percpu *dev_id)
 {
 	struct irqaction *action;
 	struct irq_desc *desc;
@@ -1981,12 +1983,15 @@  int request_percpu_irq(unsigned int irq, irq_handler_t handler,
 	    !irq_settings_is_per_cpu_devid(desc))
 		return -EINVAL;
 
+	if (flags && flags != IRQF_TIMER)
+		return -EINVAL;
+
 	action = kzalloc(sizeof(struct irqaction), GFP_KERNEL);
 	if (!action)
 		return -ENOMEM;
 
 	action->handler = handler;
-	action->flags = IRQF_PERCPU | IRQF_NO_SUSPEND;
+	action->flags = flags | IRQF_PERCPU | IRQF_NO_SUSPEND;
 	action->name = devname;
 	action->percpu_dev_id = dev_id;
 
@@ -2007,7 +2012,7 @@  int request_percpu_irq(unsigned int irq, irq_handler_t handler,
 
 	return retval;
 }
-EXPORT_SYMBOL_GPL(request_percpu_irq);
+EXPORT_SYMBOL_GPL(request_percpu_irq_flags);
 
 /**
  *	irq_get_irqchip_state - returns the irqchip state of a interrupt.
diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 35d7100..602e0a8 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -523,8 +523,9 @@  int kvm_timer_hyp_init(void)
 		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;
 	}
 
-	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,
-				 "kvm guest timer", kvm_get_running_vcpus());
+	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,
+				       IRQF_TIMER, "kvm guest timer",
+				       kvm_get_running_vcpus());
 	if (err) {
 		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",
 			host_vtimer_irq, err);