| Message ID | 1505470561-21982-1-git-send-email-leo.yan@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] doc: coresight: correct usage for disabling idle states | expand |
On 15 September 2017 at 04:16, Leo Yan <leo.yan@linaro.org> wrote: > In the coresight CPU debug document it suggests to use 'echo' command > to set latency request to /dev/cpu_dma_latency so can disable all CPU > idle states, but in fact this doesn't work. > > This is because when the command 'echo' exits, it releases the device > node's file descriptor and the kernel release function removes the QoS > constraint; finally when the command 'echo' finished there have no > constraint imposed on cpu_dma_latency. > > This patch changes to use 'exec' to access '/dev/cpu_dma_latency', the > command 'exec' can avoid the file descriptor to be closed so we can > keep the constraint on cpu_dma_latency. > > This patch also adds the info for reference docs for PM QoS and cpuidle > sysfs. > > Cc: Jonathan Corbet <corbet@lwn.net> > Cc: Mathieu Poirier <mathieu.poirier@linaro.org> > Cc: Sudeep Holla <sudeep.holla@arm.com> > Reported-by: Kim Phillips <kim.phillips@arm.com> > Signed-off-by: Leo Yan <leo.yan@linaro.org> > --- > Documentation/trace/coresight-cpu-debug.txt | 14 +++++++++----- > 1 file changed, 9 insertions(+), 5 deletions(-) > > diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt > index b3da1f9..205ff95 100644 > --- a/Documentation/trace/coresight-cpu-debug.txt > +++ b/Documentation/trace/coresight-cpu-debug.txt > @@ -149,11 +149,15 @@ If you want to limit idle states at boot time, you can use "nohlt" or > > At the runtime you can disable idle states with below methods: > > -Set latency request to /dev/cpu_dma_latency to disable all CPUs specific idle > -states (if latency = 0uS then disable all idle states): > -# echo "what_ever_latency_you_need_in_uS" > /dev/cpu_dma_latency > - > -Disable specific CPU's specific idle state: > +By using PM QoS interface '/dev/cpu_dma_latency', we can set latency > +constraint to disable all CPUs specific idle states (see > +Documentation/power/pm_qos_interface.txt, section 'From user mode'); > +below is one example to set latency constraint to '00000000', it is > +hexadecimal format with microsecond unit: > +# exec 3<> /dev/cpu_dma_latency; echo '00000000' >&3 Since doing echo '00000000' >&3 or simply echo 0 >&3 yields the same result I would go for the latter. I also think it is important to specify that using an "echo" command without holding the file open won't give the desired result. I would reformat your paragraph as follow: >>> Begin >>> It is possible to disable CPU idle states by way of the PM QoS subsystem, more specifically by using the "/dev/cpu_dma_latency" interface (see Documentation/power/pm_qos_interface.txt for more details). As specified in the PM QoS documentation the requested parameter will stay in effect until the file descriptor is released. For example: # exec 3<> /dev/cpu_dma_latency; echo 0 >&3 ... Do some work... ... # exec 3<>- The same can also be done from an application program. <<< End <<< > + > +Disable specific CPU's specific idle state from cpuidle sysfs (see > +Documentation/cpuidle/sysfs.txt): > # echo 1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable > > > -- > 2.7.4 >
On Tue, Sep 19, 2017 at 03:32:54PM -0600, Mathieu Poirier wrote: > On 15 September 2017 at 04:16, Leo Yan <leo.yan@linaro.org> wrote: > > In the coresight CPU debug document it suggests to use 'echo' command > > to set latency request to /dev/cpu_dma_latency so can disable all CPU > > idle states, but in fact this doesn't work. > > > > This is because when the command 'echo' exits, it releases the device > > node's file descriptor and the kernel release function removes the QoS > > constraint; finally when the command 'echo' finished there have no > > constraint imposed on cpu_dma_latency. > > > > This patch changes to use 'exec' to access '/dev/cpu_dma_latency', the > > command 'exec' can avoid the file descriptor to be closed so we can > > keep the constraint on cpu_dma_latency. > > > > This patch also adds the info for reference docs for PM QoS and cpuidle > > sysfs. > > > > Cc: Jonathan Corbet <corbet@lwn.net> > > Cc: Mathieu Poirier <mathieu.poirier@linaro.org> > > Cc: Sudeep Holla <sudeep.holla@arm.com> > > Reported-by: Kim Phillips <kim.phillips@arm.com> > > Signed-off-by: Leo Yan <leo.yan@linaro.org> > > --- > > Documentation/trace/coresight-cpu-debug.txt | 14 +++++++++----- > > 1 file changed, 9 insertions(+), 5 deletions(-) > > > > diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt > > index b3da1f9..205ff95 100644 > > --- a/Documentation/trace/coresight-cpu-debug.txt > > +++ b/Documentation/trace/coresight-cpu-debug.txt > > @@ -149,11 +149,15 @@ If you want to limit idle states at boot time, you can use "nohlt" or > > > > At the runtime you can disable idle states with below methods: > > > > -Set latency request to /dev/cpu_dma_latency to disable all CPUs specific idle > > -states (if latency = 0uS then disable all idle states): > > -# echo "what_ever_latency_you_need_in_uS" > /dev/cpu_dma_latency > > - > > -Disable specific CPU's specific idle state: > > +By using PM QoS interface '/dev/cpu_dma_latency', we can set latency > > +constraint to disable all CPUs specific idle states (see > > +Documentation/power/pm_qos_interface.txt, section 'From user mode'); > > +below is one example to set latency constraint to '00000000', it is > > +hexadecimal format with microsecond unit: > > +# exec 3<> /dev/cpu_dma_latency; echo '00000000' >&3 > > Since doing echo '00000000' >&3 or simply echo 0 >&3 yields the same > result I would go for the latter. I also think it is important to > specify that using an "echo" command without holding the file open > won't give the desired result. I would reformat your paragraph as > follow: > > >>> Begin >>> > > It is possible to disable CPU idle states by way of the PM QoS > subsystem, more specifically by using the "/dev/cpu_dma_latency" > interface (see Documentation/power/pm_qos_interface.txt for more > details). As specified in the PM QoS documentation the requested > parameter will stay in effect until the file descriptor is released. > For example: > > # exec 3<> /dev/cpu_dma_latency; echo 0 >&3 > ... > Do some work... > ... > # exec 3<>- > > The same can also be done from an application program. > > <<< End <<< Very appreciate your rephasing and reviewing. Will spin a new patch with it. Thanks, Leo Yan > > + > > +Disable specific CPU's specific idle state from cpuidle sysfs (see > > +Documentation/cpuidle/sysfs.txt): > > # echo 1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable > > > > > > -- > > 2.7.4 > >
diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt index b3da1f9..205ff95 100644 --- a/Documentation/trace/coresight-cpu-debug.txt +++ b/Documentation/trace/coresight-cpu-debug.txt @@ -149,11 +149,15 @@ If you want to limit idle states at boot time, you can use "nohlt" or At the runtime you can disable idle states with below methods: -Set latency request to /dev/cpu_dma_latency to disable all CPUs specific idle -states (if latency = 0uS then disable all idle states): -# echo "what_ever_latency_you_need_in_uS" > /dev/cpu_dma_latency - -Disable specific CPU's specific idle state: +By using PM QoS interface '/dev/cpu_dma_latency', we can set latency +constraint to disable all CPUs specific idle states (see +Documentation/power/pm_qos_interface.txt, section 'From user mode'); +below is one example to set latency constraint to '00000000', it is +hexadecimal format with microsecond unit: +# exec 3<> /dev/cpu_dma_latency; echo '00000000' >&3 + +Disable specific CPU's specific idle state from cpuidle sysfs (see +Documentation/cpuidle/sysfs.txt): # echo 1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable
In the coresight CPU debug document it suggests to use 'echo' command to set latency request to /dev/cpu_dma_latency so can disable all CPU idle states, but in fact this doesn't work. This is because when the command 'echo' exits, it releases the device node's file descriptor and the kernel release function removes the QoS constraint; finally when the command 'echo' finished there have no constraint imposed on cpu_dma_latency. This patch changes to use 'exec' to access '/dev/cpu_dma_latency', the command 'exec' can avoid the file descriptor to be closed so we can keep the constraint on cpu_dma_latency. This patch also adds the info for reference docs for PM QoS and cpuidle sysfs. Cc: Jonathan Corbet <corbet@lwn.net> Cc: Mathieu Poirier <mathieu.poirier@linaro.org> Cc: Sudeep Holla <sudeep.holla@arm.com> Reported-by: Kim Phillips <kim.phillips@arm.com> Signed-off-by: Leo Yan <leo.yan@linaro.org> --- Documentation/trace/coresight-cpu-debug.txt | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) -- 2.7.4