linux-kernel - Re: [PATCH 1/3] perf: use hrtimer for event multiplexing

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Message-ID: <CABPqkBT=2H5WuWZ+uFq39XTTxniVvT6tEuaYDHvaV2VSbjBPDQ@mail.gmail.com>
Date:	Fri, 7 Sep 2012 19:03:28 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...ux.intel.com,
	zheng.z.yan@...el.com, robert.richter@....com
Subject: Re: [PATCH 1/3] perf: use hrtimer for event multiplexing

On Fri, Sep 7, 2012 at 5:39 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, 2012-09-07 at 16:29 +0200, Stephane Eranian wrote:
>> @@ -148,6 +148,15 @@ static LIST_HEAD(pmus);
>>  static DEFINE_MUTEX(pmus_lock);
>>  static struct srcu_struct pmus_srcu;
>>
>> +struct perf_cpu_hrtimer {
>> +       struct hrtimer hrtimer;
>> +       int active;
>> +};
>> +
>> +static DEFINE_PER_CPU(struct list_head, rotation_list);
>> +
>> +static DEFINE_PER_CPU(struct perf_cpu_hrtimer, perf_cpu_hrtimer);
>
>
> How about sticking the hrtimer in perf_cpu_context so you can have a
> different rotation interval per PMU ?
>
I think having different intervals would be a good thing, especially for uncore.
But now, I am wondering how this could work without too much overhead.
Looks like you're suggesting arming multiple hrtimers if multiple PMU are
overcommitted. Is that right? As opposed to having a PMU multiplier off of a
single per-cpu hrtimer.

> Sorta like e9d2b064149ff7ef4acbc65a1b9374ac8b218d3e removed. Stopping
> the timer when the PMU isn't over committed should solve the NOHZ
> problem I think.

Yeah, my patch does solve this. That's what I show in my example in the intro
msg.
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