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

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Date:	Wed, 12 Sep 2012 16:22:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...ux.intel.com,
	zheng.z.yan@...el.com, robert.richter@....com
Subject: Re: [PATCH v2 2/3] perf: use hrtimer for event multiplexing

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?

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