linux-kernel - Re: [PATCH v2 2/3] perf: use hrtimer for event multiplexing

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Date:	Wed, 12 Sep 2012 17:49:41 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>,
	Robert Richter <robert.richter@....com>
Subject: Re: [PATCH v2 2/3] perf: use hrtimer for event multiplexing

On Wed, Sep 12, 2012 at 5:37 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Wed, Sep 12, 2012 at 4:43 PM, Stephane Eranian <eranian@...gle.com> wrote:
>> On Wed, Sep 12, 2012 at 4:22 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> On Wed, 2012-09-12 at 16:13 +0200, Stephane Eranian wrote:
>>>> +static DEFINE_PER_CPU(struct list_head, rotation_list);
>>>
>>> Why do you keep the rotation list? The only use seems to be:
>>>
>>>
>>>> +void perf_cpu_hrtimer_cancel(int cpu)
>>>> +{
>>>> +       struct list_head *head = &__get_cpu_var(rotation_list);
>>>> +       struct perf_cpu_context *cpuctx, *tmp;
>>>> +       unsigned long flags;
>>>> +
>>>> +       if (WARN_ON(cpu != smp_processor_id()))
>>>> +               return;
>>>> +
>>>> +       local_irq_save(flags);
>>>> +
>>>> +       list_for_each_entry_safe(cpuctx, tmp, head, rotation_list) {
>>>> +               if (cpuctx->hrtimer_active) {
>>>> +                       hrtimer_cancel(&cpuctx->hrtimer);
>>>> +                       cpuctx->hrtimer_active = 0;
>>>> +               }
>>>> +       }
>>>> +
>>>> +       local_irq_restore(flags);
>>>> +}
>>>
>>> Which is weird, why not use the existing for-each-pmu loop in
>>> perf_event_exit_cpu_context() ? Or something similar to iterate all
>>> extant PMUs and thus their cpuctxs?
>>>
>> True. That would probably work too.
>>
> Note however that the rotation_list is still used in perf_event_task_tick()
> to iterate over the ctx which needs unthrottling. We would have to switch
> that loop over to a for-each-pmu() which would necessary incur more
> iterations as it would include all the SW PMUs.

That reminds me that dropping sw context from rotation_list causes an
issue in perf_event_task_tick() because that means the sw PMU are
not considered anymore for interrupt unthrottling but they should. So
I think switching to for-each-pmu() in perf_event_task_tick() will solve
that problem too.
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