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

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Date:	Wed, 12 Sep 2012 17:37:18 +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 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.
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