linux-kernel - Re: [RFC][PATCH] perf: Rewrite core context handling

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Date:   Thu, 18 Oct 2018 10:05:11 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        songliubraving@...com, eranian@...gle.com, tglx@...utronix.de,
        mark.rutland@....com, megha.dey@...el.com, frederic@...nel.org
Subject: Re: [RFC][PATCH] perf: Rewrite core context handling

Hi,

On 17.10.2018 19:30, Peter Zijlstra wrote:
> On Wed, Oct 17, 2018 at 11:57:49AM +0300, Alexey Budankov wrote:
>> Hi,
>>
>> On 10.10.2018 13:45, Peter Zijlstra wrote:
>> <SNIP>
>>> -static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
>>> +/*
>>> + * XXX somewhat completely buggered; this is in cpu_pmu_context, but we need
>>> + * event_pmu_context for rotations. We also need event_pmu_context specific
>>> + * scheduling routines. ARGH
>>> + *
>>> + *  - fixed the cpu_pmu_context vs event_pmu_context thingy
>>> + *    (cpu_pmu_context embeds an event_pmu_context)
>>> + *
>>> + *  - need nr_events/nr_active in epc to do per epc rotation
>>> + *    (done)
>>> + *
>>> + *  - need cpu and task pmu ctx together...
>>> + *    (cpc->task_epc)
>>> + */
>>> +static bool perf_rotate_context(struct perf_cpu_pmu_context *cpc)
>>
>> Since it reduces to single cpu context (and single task context) at all times, 
>> ideally, it would probably be coded as simple as this: 
>>
>> 	perf_rotate_context()
>> 	{
>>             cpu = this_cpu_ptr(&cpu_context)
>>             for_every_pmu(pmu, cpu)
> 
> Can't do that, because we have per PMU rotation periods..

Well, yes, the callback is already called per-cpu per-pmu, 
so then this simplifies a bit, like this:

perf_rotate_context(pmu, cpu)
{
	for_every_event_ctx(event_ctx, pmu)
		rotate(event_ctx, pmu)
}


                                        event_ctx
                                           |
                                           v
    pmu (struct perf_cpu_pmu_context) ->  ctx__0 -> ctx__1
                                           |         |
                                           v         v
                           sched_out -> fgroup00  fgroup01 -> event001 -> event101 -> event201
                                          |  ^      |  ^
                                          v  |      v  |
                                        fgroup10  fgroup11
                                          |  |      |  |
                                          v  |      v  |
                            sched_in -> fgroup20  fgroup21

> 
>>                     for_every_event_ctx(event_ctx, pmu)
>> 	                    rotate(event_ctx, pmu)
>> 	}
> 
> I'm also not sure I get the rest that follows... you only have to rotate
> _one_ event per PMU.

Yes. One group per PMU. It could end up in several HW counters reprogramming.

Thanks,
Alexey

> 
> I'll try and understand the rest of you email later; brain has checked
> out for the day.
>