Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Namhyung Kim]

Hi Peter and Kan,

(Adding PPC folks)

On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> Hello,
>
> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
> >
> >
> >
> > On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
> > > On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
> > >
> > >> - When the large PEBS was introduced (9c964efa4330), the sched_task() should
> > >> be invoked to flush the PEBS buffer in each context switch. However, The
> > >> perf_sched_events in account_event() is not updated accordingly. The
> > >> perf_event_task_sched_* never be invoked for a pure per-CPU context. Only
> > >> per-task event works.
> > >>     At that time, the perf_pmu_sched_task() is outside of
> > >> perf_event_context_sched_in/out. It means that perf has to double
> > >> perf_pmu_disable() for per-task event.
> > >
> > >> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
> > >> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
> > >> sched_cb_list. Yes, the code is very similar to the original codes, but it
> > >> is actually the new code for per-CPU events. The optimization for per-task
> > >> events is still kept.
> > >>    For the case, which has both a CPU context and a task context, yes, the
> > >> __perf_pmu_sched_task() in this patch is not invoked. Because the
> > >> sched_task() only need to be invoked once in a context switch. The
> > >> sched_task() will be eventually invoked in the task context.
> > >
> > > The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
> > > only set that for large pebs. Are you sure the other users (Intel LBR
> > > and PowerPC BHRB) don't need it?
> >
> > I didn't set it for LBR, because the perf_sched_events is always enabled
> > for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
> > for LBR.
> >
> >         if (has_branch_stack(event))
> >                 inc = true;
> >
> > >
> > > If they indeed do not require the pmu::sched_task() callback for CPU
> > > events, then I still think the whole perf_sched_cb_{inc,dec}() interface
> >
> > No, LBR requires the pmu::sched_task() callback for CPU events.
> >
> > Now, The LBR registers have to be reset in sched in even for CPU events.
> >
> > To fix the shorter LBR callstack issue for CPU events, we also need to
> > save/restore LBRs in pmu::sched_task().
> > https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@linux.intel.com/
> >
> > > is confusing at best.
> > >
> > > Can't we do something like this instead?
> > >
> > I think the below patch may have two issues.
> > - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well) now.
> > - We may disable the large PEBS later if not all PEBS events support
> > large PEBS. The PMU need a way to notify the generic code to decrease
> > the nr_sched_task.
>
> Any updates on this?  I've reviewed and tested Kan's patches
> and they all look good.
>
> Maybe we can talk to PPC folks to confirm the BHRB case?

Can we move this forward?  I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
for PowerPC too.  But it'd be nice if ppc folks can confirm the change.

Thanks,
Namhyung

Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Michael Ellerman]

Namhyung Kim <namhyung@kernel.org> writes:
> Hi Peter and Kan,
>
> (Adding PPC folks)
>
> On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
>>
>> Hello,
>>
>> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>> >
>> >
>> >
>> > On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
>> > > On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
>> > >
>> > >> - When the large PEBS was introduced (9c964efa4330), the sched_task() should
>> > >> be invoked to flush the PEBS buffer in each context switch. However, The
>> > >> perf_sched_events in account_event() is not updated accordingly. The
>> > >> perf_event_task_sched_* never be invoked for a pure per-CPU context. Only
>> > >> per-task event works.
>> > >>     At that time, the perf_pmu_sched_task() is outside of
>> > >> perf_event_context_sched_in/out. It means that perf has to double
>> > >> perf_pmu_disable() for per-task event.
>> > >
>> > >> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
>> > >> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
>> > >> sched_cb_list. Yes, the code is very similar to the original codes, but it
>> > >> is actually the new code for per-CPU events. The optimization for per-task
>> > >> events is still kept.
>> > >>    For the case, which has both a CPU context and a task context, yes, the
>> > >> __perf_pmu_sched_task() in this patch is not invoked. Because the
>> > >> sched_task() only need to be invoked once in a context switch. The
>> > >> sched_task() will be eventually invoked in the task context.
>> > >
>> > > The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
>> > > only set that for large pebs. Are you sure the other users (Intel LBR
>> > > and PowerPC BHRB) don't need it?
>> >
>> > I didn't set it for LBR, because the perf_sched_events is always enabled
>> > for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
>> > for LBR.
>> >
>> >         if (has_branch_stack(event))
>> >                 inc = true;
>> >
>> > >
>> > > If they indeed do not require the pmu::sched_task() callback for CPU
>> > > events, then I still think the whole perf_sched_cb_{inc,dec}() interface
>> >
>> > No, LBR requires the pmu::sched_task() callback for CPU events.
>> >
>> > Now, The LBR registers have to be reset in sched in even for CPU events.
>> >
>> > To fix the shorter LBR callstack issue for CPU events, we also need to
>> > save/restore LBRs in pmu::sched_task().
>> > https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@linux.intel.com/
>> >
>> > > is confusing at best.
>> > >
>> > > Can't we do something like this instead?
>> > >
>> > I think the below patch may have two issues.
>> > - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well) now.
>> > - We may disable the large PEBS later if not all PEBS events support
>> > large PEBS. The PMU need a way to notify the generic code to decrease
>> > the nr_sched_task.
>>
>> Any updates on this?  I've reviewed and tested Kan's patches
>> and they all look good.
>>
>> Maybe we can talk to PPC folks to confirm the BHRB case?
>
> Can we move this forward?  I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
> for PowerPC too.  But it'd be nice if ppc folks can confirm the change.

Sorry I've read the whole thread, but I'm still not entirely sure I
understand the question.

cheers

Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Namhyung Kim]

Hello,

On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <mpe@ellerman.id.au> wrote:
>
> Namhyung Kim <namhyung@kernel.org> writes:
> > Hi Peter and Kan,
> >
> > (Adding PPC folks)
> >
> > On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >>
> >> Hello,
> >>
> >> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
> >> >
> >> >
> >> >
> >> > On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
> >> > > On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
> >> > >
> >> > >> - When the large PEBS was introduced (9c964efa4330), the sched_task() should
> >> > >> be invoked to flush the PEBS buffer in each context switch. However, The
> >> > >> perf_sched_events in account_event() is not updated accordingly. The
> >> > >> perf_event_task_sched_* never be invoked for a pure per-CPU context. Only
> >> > >> per-task event works.
> >> > >>     At that time, the perf_pmu_sched_task() is outside of
> >> > >> perf_event_context_sched_in/out. It means that perf has to double
> >> > >> perf_pmu_disable() for per-task event.
> >> > >
> >> > >> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
> >> > >> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
> >> > >> sched_cb_list. Yes, the code is very similar to the original codes, but it
> >> > >> is actually the new code for per-CPU events. The optimization for per-task
> >> > >> events is still kept.
> >> > >>    For the case, which has both a CPU context and a task context, yes, the
> >> > >> __perf_pmu_sched_task() in this patch is not invoked. Because the
> >> > >> sched_task() only need to be invoked once in a context switch. The
> >> > >> sched_task() will be eventually invoked in the task context.
> >> > >
> >> > > The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
> >> > > only set that for large pebs. Are you sure the other users (Intel LBR
> >> > > and PowerPC BHRB) don't need it?
> >> >
> >> > I didn't set it for LBR, because the perf_sched_events is always enabled
> >> > for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
> >> > for LBR.
> >> >
> >> >         if (has_branch_stack(event))
> >> >                 inc = true;
> >> >
> >> > >
> >> > > If they indeed do not require the pmu::sched_task() callback for CPU
> >> > > events, then I still think the whole perf_sched_cb_{inc,dec}() interface
> >> >
> >> > No, LBR requires the pmu::sched_task() callback for CPU events.
> >> >
> >> > Now, The LBR registers have to be reset in sched in even for CPU events.
> >> >
> >> > To fix the shorter LBR callstack issue for CPU events, we also need to
> >> > save/restore LBRs in pmu::sched_task().
> >> > https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@linux.intel.com/
> >> >
> >> > > is confusing at best.
> >> > >
> >> > > Can't we do something like this instead?
> >> > >
> >> > I think the below patch may have two issues.
> >> > - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well) now.
> >> > - We may disable the large PEBS later if not all PEBS events support
> >> > large PEBS. The PMU need a way to notify the generic code to decrease
> >> > the nr_sched_task.
> >>
> >> Any updates on this?  I've reviewed and tested Kan's patches
> >> and they all look good.
> >>
> >> Maybe we can talk to PPC folks to confirm the BHRB case?
> >
> > Can we move this forward?  I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
> > for PowerPC too.  But it'd be nice if ppc folks can confirm the change.
>
> Sorry I've read the whole thread, but I'm still not entirely sure I
> understand the question.

Thanks for your time and sorry about not being clear enough.

We found per-cpu events are not calling pmu::sched_task()
on context switches.  So PERF_ATTACH_SCHED_CB was
added to indicate the core logic that it needs to invoke the
callback.

The patch 3/3 added the flag to PPC (for BHRB) with other
changes (I think it should be split like in the patch 2/3) and
want to get ACKs from the PPC folks.

Thanks,
Namhyung

Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Madhavan Srinivasan]

On 11/24/20 10:21 AM, Namhyung Kim wrote:
> Hello,
>
> On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <mpe@ellerman.id.au> wrote:
>> Namhyung Kim <namhyung@kernel.org> writes:
>>> Hi Peter and Kan,
>>>
>>> (Adding PPC folks)
>>>
>>> On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
>>>> Hello,
>>>>
>>>> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>>>>
>>>>>
>>>>> On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
>>>>>> On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
>>>>>>
>>>>>>> - When the large PEBS was introduced (9c964efa4330), the sched_task() should
>>>>>>> be invoked to flush the PEBS buffer in each context switch. However, The
>>>>>>> perf_sched_events in account_event() is not updated accordingly. The
>>>>>>> perf_event_task_sched_* never be invoked for a pure per-CPU context. Only
>>>>>>> per-task event works.
>>>>>>>      At that time, the perf_pmu_sched_task() is outside of
>>>>>>> perf_event_context_sched_in/out. It means that perf has to double
>>>>>>> perf_pmu_disable() for per-task event.
>>>>>>> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
>>>>>>> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
>>>>>>> sched_cb_list. Yes, the code is very similar to the original codes, but it
>>>>>>> is actually the new code for per-CPU events. The optimization for per-task
>>>>>>> events is still kept.
>>>>>>>     For the case, which has both a CPU context and a task context, yes, the
>>>>>>> __perf_pmu_sched_task() in this patch is not invoked. Because the
>>>>>>> sched_task() only need to be invoked once in a context switch. The
>>>>>>> sched_task() will be eventually invoked in the task context.
>>>>>> The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
>>>>>> only set that for large pebs. Are you sure the other users (Intel LBR
>>>>>> and PowerPC BHRB) don't need it?
>>>>> I didn't set it for LBR, because the perf_sched_events is always enabled
>>>>> for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
>>>>> for LBR.
>>>>>
>>>>>          if (has_branch_stack(event))
>>>>>                  inc = true;
>>>>>
>>>>>> If they indeed do not require the pmu::sched_task() callback for CPU
>>>>>> events, then I still think the whole perf_sched_cb_{inc,dec}() interface
>>>>> No, LBR requires the pmu::sched_task() callback for CPU events.
>>>>>
>>>>> Now, The LBR registers have to be reset in sched in even for CPU events.
>>>>>
>>>>> To fix the shorter LBR callstack issue for CPU events, we also need to
>>>>> save/restore LBRs in pmu::sched_task().
>>>>> https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@linux.intel.com/
>>>>>
>>>>>> is confusing at best.
>>>>>>
>>>>>> Can't we do something like this instead?
>>>>>>
>>>>> I think the below patch may have two issues.
>>>>> - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well) now.
>>>>> - We may disable the large PEBS later if not all PEBS events support
>>>>> large PEBS. The PMU need a way to notify the generic code to decrease
>>>>> the nr_sched_task.
>>>> Any updates on this?  I've reviewed and tested Kan's patches
>>>> and they all look good.
>>>>
>>>> Maybe we can talk to PPC folks to confirm the BHRB case?
>>> Can we move this forward?  I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
>>> for PowerPC too.  But it'd be nice if ppc folks can confirm the change.
>> Sorry I've read the whole thread, but I'm still not entirely sure I
>> understand the question.
> Thanks for your time and sorry about not being clear enough.
>
> We found per-cpu events are not calling pmu::sched_task()
> on context switches.  So PERF_ATTACH_SCHED_CB was
> added to indicate the core logic that it needs to invoke the
> callback.
>
> The patch 3/3 added the flag to PPC (for BHRB) with other
> changes (I think it should be split like in the patch 2/3) and
> want to get ACKs from the PPC folks.

Sorry for delay.

I guess first it will be better to split the ppc change to a separate patch,

secondly, we are missing the changes needed in the power_pmu_bhrb_disable()

where perf_sched_cb_dec() needs the "state" to be included.


Maddy


>
> Thanks,
> Namhyung

Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Liang, Kan]

On 11/24/2020 12:42 AM, Madhavan Srinivasan wrote:
> 
> On 11/24/20 10:21 AM, Namhyung Kim wrote:
>> Hello,
>>
>> On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <mpe@ellerman.id.au> 
>> wrote:
>>> Namhyung Kim <namhyung@kernel.org> writes:
>>>> Hi Peter and Kan,
>>>>
>>>> (Adding PPC folks)
>>>>
>>>> On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> 
>>>> wrote:
>>>>> Hello,
>>>>>
>>>>> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan 
>>>>> <kan.liang@linux.intel.com> wrote:
>>>>>>
>>>>>>
>>>>>> On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
>>>>>>> On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
>>>>>>>
>>>>>>>> - When the large PEBS was introduced (9c964efa4330), the 
>>>>>>>> sched_task() should
>>>>>>>> be invoked to flush the PEBS buffer in each context switch. 
>>>>>>>> However, The
>>>>>>>> perf_sched_events in account_event() is not updated accordingly. 
>>>>>>>> The
>>>>>>>> perf_event_task_sched_* never be invoked for a pure per-CPU 
>>>>>>>> context. Only
>>>>>>>> per-task event works.
>>>>>>>>      At that time, the perf_pmu_sched_task() is outside of
>>>>>>>> perf_event_context_sched_in/out. It means that perf has to double
>>>>>>>> perf_pmu_disable() for per-task event.
>>>>>>>> - The patch 1 tries to fix broken per-CPU events. The CPU 
>>>>>>>> context cannot be
>>>>>>>> retrieved from the task->perf_event_ctxp. So it has to be 
>>>>>>>> tracked in the
>>>>>>>> sched_cb_list. Yes, the code is very similar to the original 
>>>>>>>> codes, but it
>>>>>>>> is actually the new code for per-CPU events. The optimization 
>>>>>>>> for per-task
>>>>>>>> events is still kept.
>>>>>>>>     For the case, which has both a CPU context and a task 
>>>>>>>> context, yes, the
>>>>>>>> __perf_pmu_sched_task() in this patch is not invoked. Because the
>>>>>>>> sched_task() only need to be invoked once in a context switch. The
>>>>>>>> sched_task() will be eventually invoked in the task context.
>>>>>>> The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB 
>>>>>>> and
>>>>>>> only set that for large pebs. Are you sure the other users (Intel 
>>>>>>> LBR
>>>>>>> and PowerPC BHRB) don't need it?
>>>>>> I didn't set it for LBR, because the perf_sched_events is always 
>>>>>> enabled
>>>>>> for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
>>>>>> for LBR.
>>>>>>
>>>>>>          if (has_branch_stack(event))
>>>>>>                  inc = true;
>>>>>>
>>>>>>> If they indeed do not require the pmu::sched_task() callback for CPU
>>>>>>> events, then I still think the whole perf_sched_cb_{inc,dec}() 
>>>>>>> interface
>>>>>> No, LBR requires the pmu::sched_task() callback for CPU events.
>>>>>>
>>>>>> Now, The LBR registers have to be reset in sched in even for CPU 
>>>>>> events.
>>>>>>
>>>>>> To fix the shorter LBR callstack issue for CPU events, we also 
>>>>>> need to
>>>>>> save/restore LBRs in pmu::sched_task().
>>>>>> https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@linux.intel.com/ 
>>>>>>
>>>>>>
>>>>>>> is confusing at best.
>>>>>>>
>>>>>>> Can't we do something like this instead?
>>>>>>>
>>>>>> I think the below patch may have two issues.
>>>>>> - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as 
>>>>>> well) now.
>>>>>> - We may disable the large PEBS later if not all PEBS events support
>>>>>> large PEBS. The PMU need a way to notify the generic code to decrease
>>>>>> the nr_sched_task.
>>>>> Any updates on this?  I've reviewed and tested Kan's patches
>>>>> and they all look good.
>>>>>
>>>>> Maybe we can talk to PPC folks to confirm the BHRB case?
>>>> Can we move this forward?  I saw patch 3/3 also adds 
>>>> PERF_ATTACH_SCHED_CB
>>>> for PowerPC too.  But it'd be nice if ppc folks can confirm the change.
>>> Sorry I've read the whole thread, but I'm still not entirely sure I
>>> understand the question.
>> Thanks for your time and sorry about not being clear enough.
>>
>> We found per-cpu events are not calling pmu::sched_task()
>> on context switches.  So PERF_ATTACH_SCHED_CB was
>> added to indicate the core logic that it needs to invoke the
>> callback.
>>
>> The patch 3/3 added the flag to PPC (for BHRB) with other
>> changes (I think it should be split like in the patch 2/3) and
>> want to get ACKs from the PPC folks.
> 
> Sorry for delay.
> 
> I guess first it will be better to split the ppc change to a separate 
> patch,

Both PPC and X86 invokes the perf_sched_cb_inc() directly. The patch 
changes the parameters of the perf_sched_cb_inc(). I think we have to 
update the PPC and X86 codes together. Otherwise, there will be a 
compile error, if someone may only applies the change for the 
perf_sched_cb_inc() but forget to applies the changes in PPC or X86 
specific codes.

> 
> secondly, we are missing the changes needed in the power_pmu_bhrb_disable()
> 
> where perf_sched_cb_dec() needs the "state" to be included.
> 

Ah, right. The below patch should fix the issue.

diff --git a/arch/powerpc/perf/core-book3s.c 
b/arch/powerpc/perf/core-book3s.c
index bced502f64a1..6756d1602a67 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -391,13 +391,18 @@ static void power_pmu_bhrb_enable(struct 
perf_event *event)
  static void power_pmu_bhrb_disable(struct perf_event *event)
  {
  	struct cpu_hw_events *cpuhw = this_cpu_ptr(&cpu_hw_events);
+	int state = PERF_SCHED_CB_SW_IN;

  	if (!ppmu->bhrb_nr)
  		return;

  	WARN_ON_ONCE(!cpuhw->bhrb_users);
  	cpuhw->bhrb_users--;
-	perf_sched_cb_dec(event->ctx->pmu);
+
+	if (!(event->attach_state & PERF_ATTACH_TASK))
+		state |= PERF_SCHED_CB_CPU;
+
+	perf_sched_cb_dec(event->ctx->pmu, state);

  	if (!cpuhw->disabled && !cpuhw->bhrb_users) {
  		/* BHRB cannot be turned off when other



Thanks,
Kan

Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Michael Ellerman]

Namhyung Kim <namhyung@kernel.org> writes:
> Hello,
>
> On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <mpe@ellerman.id.au> wrote:
>>
>> Namhyung Kim <namhyung@kernel.org> writes:
>> > Hi Peter and Kan,
>> >
>> > (Adding PPC folks)
>> >
>> > On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
>> >>
>> >> Hello,
>> >>
>> >> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>> >> >
>> >> >
>> >> >
>> >> > On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
>> >> > > On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
>> >> > >
>> >> > >> - When the large PEBS was introduced (9c964efa4330), the sched_task() should
>> >> > >> be invoked to flush the PEBS buffer in each context switch. However, The
>> >> > >> perf_sched_events in account_event() is not updated accordingly. The
>> >> > >> perf_event_task_sched_* never be invoked for a pure per-CPU context. Only
>> >> > >> per-task event works.
>> >> > >>     At that time, the perf_pmu_sched_task() is outside of
>> >> > >> perf_event_context_sched_in/out. It means that perf has to double
>> >> > >> perf_pmu_disable() for per-task event.
>> >> > >
>> >> > >> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
>> >> > >> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
>> >> > >> sched_cb_list. Yes, the code is very similar to the original codes, but it
>> >> > >> is actually the new code for per-CPU events. The optimization for per-task
>> >> > >> events is still kept.
>> >> > >>    For the case, which has both a CPU context and a task context, yes, the
>> >> > >> __perf_pmu_sched_task() in this patch is not invoked. Because the
>> >> > >> sched_task() only need to be invoked once in a context switch. The
>> >> > >> sched_task() will be eventually invoked in the task context.
>> >> > >
>> >> > > The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
>> >> > > only set that for large pebs. Are you sure the other users (Intel LBR
>> >> > > and PowerPC BHRB) don't need it?
>> >> >
>> >> > I didn't set it for LBR, because the perf_sched_events is always enabled
>> >> > for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
>> >> > for LBR.
>> >> >
>> >> >         if (has_branch_stack(event))
>> >> >                 inc = true;
>> >> >
>> >> > >
>> >> > > If they indeed do not require the pmu::sched_task() callback for CPU
>> >> > > events, then I still think the whole perf_sched_cb_{inc,dec}() interface
>> >> >
>> >> > No, LBR requires the pmu::sched_task() callback for CPU events.
>> >> >
>> >> > Now, The LBR registers have to be reset in sched in even for CPU events.
>> >> >
>> >> > To fix the shorter LBR callstack issue for CPU events, we also need to
>> >> > save/restore LBRs in pmu::sched_task().
>> >> > https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@linux.intel.com/
>> >> >
>> >> > > is confusing at best.
>> >> > >
>> >> > > Can't we do something like this instead?
>> >> > >
>> >> > I think the below patch may have two issues.
>> >> > - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well) now.
>> >> > - We may disable the large PEBS later if not all PEBS events support
>> >> > large PEBS. The PMU need a way to notify the generic code to decrease
>> >> > the nr_sched_task.
>> >>
>> >> Any updates on this?  I've reviewed and tested Kan's patches
>> >> and they all look good.
>> >>
>> >> Maybe we can talk to PPC folks to confirm the BHRB case?
>> >
>> > Can we move this forward?  I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
>> > for PowerPC too.  But it'd be nice if ppc folks can confirm the change.
>>
>> Sorry I've read the whole thread, but I'm still not entirely sure I
>> understand the question.
>
> Thanks for your time and sorry about not being clear enough.
>
> We found per-cpu events are not calling pmu::sched_task()
> on context switches.  So PERF_ATTACH_SCHED_CB was
> added to indicate the core logic that it needs to invoke the
> callback.

OK. TBH I've never thought of using branch stack with a per-cpu event,
but I guess you can do it.

I think the same logic applies as LBR, we need to read the BHRB entries
in the context of the task that they were recorded for.

> The patch 3/3 added the flag to PPC (for BHRB) with other
> changes (I think it should be split like in the patch 2/3) and
> want to get ACKs from the PPC folks.

If you post a new version with Maddy's comments addressed then he or I
can ack it.

cheers