Re: [PATCH 2/7] perf: Generalize task_function_call()ers

From	Alexander Shishkin <alexander.shishkin@linux.intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 2/7] perf: Generalize task_function_call()ers
Date	2015-12-21 15:50 +0100
Message-ID	<qI9WF-7IE-9@gated-at.bofh.it> (permalink)
References	(4 earlier) <qDtMm-4Ra-23@gated-at.bofh.it> <qGH6q-8ue-15@gated-at.bofh.it> <qGHJ8-Ah-17@gated-at.bofh.it> <qGIlQ-154-27@gated-at.bofh.it> <qGZd0-3FA-31@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Peter Zijlstra <peterz@infradead.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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Re: [PATCH 2/7] perf: Generalize task_function_call()ers Peter Zijlstra <peterz@infradead.org> - 2015-12-17 14:50 +0100
  Re: [PATCH 2/7] perf: Generalize task_function_call()ers Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2015-12-17 15:30 +0100
    Re: [PATCH 2/7] perf: Generalize task_function_call()ers Peter Zijlstra <peterz@infradead.org> - 2015-12-17 16:10 +0100
      Re: [PATCH 2/7] perf: Generalize task_function_call()ers Peter Zijlstra <peterz@infradead.org> - 2015-12-18 10:10 +0100
        Re: [PATCH 2/7] perf: Generalize task_function_call()ers Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2015-12-18 16:10 +0100
          Re: [PATCH 2/7] perf: Generalize task_function_call()ers Peter Zijlstra <peterz@infradead.org> - 2015-12-18 17:50 +0100
            Re: [PATCH 2/7] perf: Generalize task_function_call()ers Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2015-12-18 18:50 +0100
        Re: [PATCH 2/7] perf: Generalize task_function_call()ers Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2015-12-21 15:50 +0100

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