linux-kernel - Re: [PATCH v9] perf: Sharing PMU counters across compatible events

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Message-ID: <CDB64D32-4E89-4884-88B8-66DCB1DFA8E3@fb.com>
Date:   Thu, 16 Jan 2020 23:59:13 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     open list <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v9] perf: Sharing PMU counters across compatible events

Hi Peter, 

> On Jan 10, 2020, at 9:37 AM, Song Liu <songliubraving@...com> wrote:
> 
>>> 
>>> @@ -2242,9 +2494,9 @@ static void __perf_event_disable(struct perf_event *event,
>>> 	}
>>> 
>>> 	if (event == event->group_leader)
>>> -		group_sched_out(event, cpuctx, ctx);
>>> +		group_sched_out(event, cpuctx, ctx, true);
>>> 	else
>>> -		event_sched_out(event, cpuctx, ctx);
>>> +		event_sched_out(event, cpuctx, ctx, true);
>>> 
>>> 	perf_event_set_state(event, PERF_EVENT_STATE_OFF);
>>> }
>> 
>> So the above event_sched_out(.remove_dup) is very inconsistent with the
>> below ctx_resched(.event_add_dup).
> 
> [...]
> 
>>> @@ -2810,7 +3069,7 @@ static void __perf_event_enable(struct perf_event *event,
>>> 	if (ctx->task)
>>> 		WARN_ON_ONCE(task_ctx != ctx);
>>> 
>>> -	ctx_resched(cpuctx, task_ctx, get_event_type(event));
>>> +	ctx_resched(cpuctx, task_ctx, get_event_type(event), event);
>>> }
>>> 
>>> /*
>> 
>> We basically need:
>> 
>> * perf_event_setup_dup() after add_event_to_ctx(), but before *sched_in()
>>  - perf_install_in_context()
>>  - perf_event_enable()
>>  - inherit_event()
>> 
>> * perf_event_remove_dup() after *sched_out(), but before list_del_event()
>>  - perf_remove_from_context()
>>  - perf_event_disable()

Quick question:

For the remove_dup() path, if we do it after *_sched_out(), we will need to 
disable-then-enable the pmu for one extra time. In current version, we only
call perf_event_remove_dup() in event_sched_out(), where extra disable/enable
is not necessary. Is it a good tradeoff to add one extra disable-enable for 
cleaner code?

Thanks,
Song