linux-kernel - Re: Perf: event wakeup discards sched

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Message-ID: <20190114130957.GA10486@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Jan 2019 14:09:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Perf: event wakeup discards sched_waking events

On Thu, Jan 10, 2019 at 12:45:27PM -0500, Mathieu Desnoyers wrote:
 
> static void perf_pending_event(struct irq_work *entry)
> {
>         struct perf_event *event = container_of(entry,
>                         struct perf_event, pending);
>         int rctx;
> 
>         rctx = perf_swevent_get_recursion_context();
>         /*
>          * If we 'fail' here, that's OK, it means recursion is already disabled
>          * and we won't recurse 'further'.
>          */
> 
>         if (event->pending_disable) {
>                 event->pending_disable = 0;
>                 perf_event_disable_local(event);
>         }
> 
>         if (event->pending_wakeup) {
>                 event->pending_wakeup = 0;
>                 perf_event_wakeup(event);
>         }
> 
>         if (rctx >= 0)
>                 perf_swevent_put_recursion_context(rctx);
> }
> 
> One side-effect of perf_event_wakeup() is to generate a sched_waking
> event. But I suspect it won't be traced by perf because it is invoked before
> putting the recursion context.
> 
> Is there a reason why the wakeup is done before putting the recursion
> context ?

d525211f9d1b ("perf: Fix irq_work 'tail' recursion")

If we were to allow perf_event_wakeup() to generate its tracepoint, we'd
be back to square #1, no?