linux-kernel - Re: [PATCH 2/7] perf: Generalize task_function

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Message-ID: <87vb88yjs6.fsf@ashishki-desk.ger.corp.intel.com>
Date:	Tue, 08 Dec 2015 18:42:01 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com, johannes@...solutions.net,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH 2/7] perf: Generalize task_function_call()ers

Peter Zijlstra <peterz@...radead.org> writes:

> OK, so the retry_state thing is clever, but either I'm too tired or its
> not quite right. Nor do I think its actually required.
>
> /me frobs...
>
> Hmm, I cannot seem to convince myself the current code is correct to
> begin with.
>
> In any case, consider the below (on top of my previous collapse patch).
> The two 'hard' cases are perf_event_{dis,en}able(), those appear to play
> silly games with event->state.
>
> So starting with perf_event_disable(); we don't strictly need to test
> for event->state == ACTIVE, ctx->is_active is enough. If the event is
> not scheduled while the ctx is, __perf_event_disable() still does the
> right thing.  Its a little less efficient to IPI in that case, over-all
> simpler.
>
> For perf_event_enable(); the same goes, but I think that's actually
> broken in its current form. The current condition is: ctx->is_active &&
> event->state == OFF, that means it doesn't do anything when !ctx->active
> && event->state == OFF. This is wrong, it should still mark the event
> INACTIVE in that case, otherwise we'll still not try and schedule the
> event once the context becomes active again.

Yes, this does look more logically correct.

>
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1766,6 +1766,20 @@ int __perf_event_disable(void *info)
>  	return 0;
>  }
>  
> +void ___perf_event_disable(void *info)

Only maybe change these to __perf_event_disable_locked() or something
visually distinctive from the 'active' callback?

FWIW,

Reviewed-by: Alexander Shishkin <alexander.shishkin@...el.com>

Thanks,
--
Alex
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