diff mbox series

[v4,4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

Message ID 20230628022554.1638318-5-longman@redhat.com
State Superseded
Headers show
Series [v4,1/4] x86/speculation: Add __update_spec_ctrl() helper | expand

Commit Message

Waiman Long June 28, 2023, 2:25 a.m. UTC
Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the cstate is 6 or lower. However, there are
some use cases where a customer may want to use max_cstate=1 to
lower latency. Such use cases will suffer from the performance
degradation caused by the enabling of IBRS in the sibling idle thread.
Add a "ibrs_off" module parameter to force disable IBRS and the
CPUIDLE_FLAG_IRQ_ENABLE flag if set.

In the case of a Skylake server with max_cstate=1, this new ibrs_off
option will likely increase the IRQ response latency as IRQ will now
be disabled.

When running SPECjbb2015 with cstates set to C1 on a Skylake system.

First test when the kernel is booted with: "intel_idle.ibrs_off"
  max-jOPS = 117828, critical-jOPS = 66047

Then retest when the kernel is booted without the "intel_idle.ibrs_off"
added.
  max-jOPS = 116408, critical-jOPS = 58958

That means booting with "intel_idle.ibrs_off" improves performance by:
  max-jOPS:   1.2%, which could be considered noise range.
  critical-jOPS: 12%, which is definitely a solid improvement.

The admin-guide/pm/intel_idle.rst file is updated to add a description
about the new "ibrs_off" module parameter.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
 drivers/idle/intel_idle.c                   | 14 ++++++++++++--
 2 files changed, 28 insertions(+), 3 deletions(-)

Comments

Waiman Long June 29, 2023, 3:03 a.m. UTC | #1
On 6/27/23 22:58, Randy Dunlap wrote:
> Hi,
>
> On 6/27/23 19:25, Waiman Long wrote:
>>   Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
>>   drivers/idle/intel_idle.c                   | 14 ++++++++++++--
>>   2 files changed, 28 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
>> index b799a43da62e..8604e6d1fe2c 100644
>> --- a/Documentation/admin-guide/pm/intel_idle.rst
>> +++ b/Documentation/admin-guide/pm/intel_idle.rst
> You may take or leave these comments as you choose.
>
>> @@ -216,6 +216,21 @@ are ignored).
>>   The idle states disabled this way can be enabled (on a per-CPU basis) from user
>>   space via ``sysfs``.
>>   
>> +The ``ibrs_off`` module parameter is a boolean flag (default to false). It is
>                                                         (default false).
> or
>                                                         (defaults to false).
>
> Then I think it reads better if the next sentence begins with: "If set,"
> instead of having it in the middle of the sentence.
>
>> +used to control if IBRS (Indirect Branch Restricted Speculation) should be
>> +turned off, if set, when the CPU enters an idle state.  This flag will not
>> +affect CPUs that are using Enhanced IBRS which can remain on with little
>> +performance impact.
>> +
>> +For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
>> +security vulnerabilities by default.  Leaving the IBRS mode on while idling may
>> +have a performance impact on its sibling CPU.  The IBRS mode will be turned off
>> +by default when the CPU enters into a deep idle state, but not in some
>> +shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
>> +mode to off when the CPU is in any one of the available idle states.  This may
>> +help performance of a sibling CPU at the expense of a slightly higher wakeup
>> +latency for the idle CPU.
> thanks.

Thanks for the suggestion. I will make the change if I have to revise 
the patch again for any reason.

Cheers,
Longman
Peter Zijlstra July 3, 2023, 10:32 a.m. UTC | #2
On Tue, Jun 27, 2023 at 10:25:54PM -0400, Waiman Long wrote:

> @@ -69,6 +69,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
>  static unsigned int disabled_states_mask __read_mostly;
>  static unsigned int preferred_states_mask __read_mostly;
>  static bool force_irq_on __read_mostly;
> +static bool ibrs_off __read_mostly;
>  
>  static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
>  
> @@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
>  	}
>  
>  	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
> -			   state->flags & CPUIDLE_FLAG_IBRS) {
> +			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
>  		/*
>  		 * IBRS mitigation requires that C-states are entered
>  		 * with interrupts disabled.
>  		 */
> -		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
> +		else
> +			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);

If you're respinning this, you can leave out the else and avoid the
indent on the WARN:

+		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);

Same effect, simpler code and all that.

>  		state->enter = intel_idle_ibrs;
>  		return;
>  	}
Waiman Long July 3, 2023, 3:09 p.m. UTC | #3
On 7/3/23 06:32, Peter Zijlstra wrote:
> On Tue, Jun 27, 2023 at 10:25:54PM -0400, Waiman Long wrote:
>
>> @@ -69,6 +69,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
>>   static unsigned int disabled_states_mask __read_mostly;
>>   static unsigned int preferred_states_mask __read_mostly;
>>   static bool force_irq_on __read_mostly;
>> +static bool ibrs_off __read_mostly;
>>   
>>   static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
>>   
>> @@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
>>   	}
>>   
>>   	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
>> -			   state->flags & CPUIDLE_FLAG_IBRS) {
>> +			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
>>   		/*
>>   		 * IBRS mitigation requires that C-states are entered
>>   		 * with interrupts disabled.
>>   		 */
>> -		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
>> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
>> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
>> +		else
>> +			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
> If you're respinning this, you can leave out the else and avoid the
> indent on the WARN:
>
> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
> 		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
>
> Same effect, simpler code and all that.

That is true. I can certainly respin that as there is another suggested 
doc change that is pending.

Cheers,
Longman
diff mbox series

Patch

diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
index b799a43da62e..8604e6d1fe2c 100644
--- a/Documentation/admin-guide/pm/intel_idle.rst
+++ b/Documentation/admin-guide/pm/intel_idle.rst
@@ -170,7 +170,7 @@  and ``idle=nomwait``.  If any of them is present in the kernel command line, the
 ``MWAIT`` instruction is not allowed to be used, so the initialization of
 ``intel_idle`` will fail.
 
-Apart from that there are four module parameters recognized by ``intel_idle``
+Apart from that there are five module parameters recognized by ``intel_idle``
 itself that can be set via the kernel command line (they cannot be updated via
 sysfs, so that is the only way to change their values).
 
@@ -216,6 +216,21 @@  are ignored).
 The idle states disabled this way can be enabled (on a per-CPU basis) from user
 space via ``sysfs``.
 
+The ``ibrs_off`` module parameter is a boolean flag (default to false). It is
+used to control if IBRS (Indirect Branch Restricted Speculation) should be
+turned off, if set, when the CPU enters an idle state.  This flag will not
+affect CPUs that are using Enhanced IBRS which can remain on with little
+performance impact.
+
+For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
+security vulnerabilities by default.  Leaving the IBRS mode on while idling may
+have a performance impact on its sibling CPU.  The IBRS mode will be turned off
+by default when the CPU enters into a deep idle state, but not in some
+shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
+mode to off when the CPU is in any one of the available idle states.  This may
+help performance of a sibling CPU at the expense of a slightly higher wakeup
+latency for the idle CPU.
+
 
 .. _intel-idle-core-and-package-idle-states:
 
diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index e32ff09051af..14874579b396 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -69,6 +69,7 @@  static int max_cstate = CPUIDLE_STATE_MAX - 1;
 static unsigned int disabled_states_mask __read_mostly;
 static unsigned int preferred_states_mask __read_mostly;
 static bool force_irq_on __read_mostly;
+static bool ibrs_off __read_mostly;
 
 static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
 
@@ -1919,12 +1920,15 @@  static void state_update_enter_method(struct cpuidle_state *state, int cstate)
 	}
 
 	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
-			   state->flags & CPUIDLE_FLAG_IBRS) {
+			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
 		/*
 		 * IBRS mitigation requires that C-states are entered
 		 * with interrupts disabled.
 		 */
-		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
+		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
+		else
+			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
 		state->enter = intel_idle_ibrs;
 		return;
 	}
@@ -2346,3 +2350,9 @@  MODULE_PARM_DESC(preferred_cstates, "Mask of preferred idle states");
  * 'CPUIDLE_FLAG_INIT_XSTATE' and 'CPUIDLE_FLAG_IBRS' flags.
  */
 module_param(force_irq_on, bool, 0444);
+/*
+ * Force the disabling of IBRS when X86_FEATURE_KERNEL_IBRS is on and
+ * CPUIDLE_FLAG_IRQ_ENABLE isn't set.
+ */
+module_param(ibrs_off, bool, 0444);
+MODULE_PARM_DESC(ibrs_off, "Disable IBRS when idle");