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

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Date:	Wed, 12 Sep 2012 16:43:38 +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 4:22 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.

> Also, you can do away with hrtimer_active, you can hrtimer_cancel() on
> an inactive hrtimer just fine, it will DTRT.

The hrtimer_active is used to prevent activating the timer multiple times
in a row.
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