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

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Date:	Mon, 21 Dec 2015 16:39:22 +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:

> -
> -	/*
> -	 * If the context we're installing events in is not the
> -	 * active task_ctx, flip them.
> -	 */
> -	if (ctx->task && task_ctx != ctx) {
> -		if (task_ctx)
> -			raw_spin_unlock(&task_ctx->lock);
> -		raw_spin_lock(&ctx->lock);
> -		task_ctx = ctx;
> -	}
> -
> -	if (task_ctx) {
> -		cpuctx->task_ctx = task_ctx;
> -		task = task_ctx->task;
> -	}
> -

So previously, this would schedule in the tast_ctx right in
perf_install_in_context path.

The new code would only reschedule the context if it is already on:

> +	if (ctx->is_active)
> +		perf_resched_context(cpuctx);
>  }

which means, iiuc, that an enabled event (say, attr.disabled==0) will
have to wait till the next time the ctx::task is scheduled instead of
getting scheduled right here.

Something like

    if (ctx->task == current && ctx->nr_events)
       perf_event_sched_in(cpuctx, ctx, ctx->task);

might make sense here.

Also the new __perf_event_enable() has the same symptom: it doesn't
schedule the new context on, only reschedule it if it's already on.

Regards,
--
Alex
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