linux-kernel - Re: [RFC PATCH] sched: Fix sched

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Date:	Fri, 5 Jun 2015 14:09:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Francis Giraldeau <francis.giraldeau@...il.com>
Subject: Re: [RFC PATCH] sched: Fix sched_wakeup tracepoint

On Fri, Jun 05, 2015 at 01:41:49PM +0200, Mathieu Desnoyers wrote:
> Commit 317f394160e9 "sched: Move the second half of ttwu() to the remote cpu"
> moves ttwu_do_wakeup() to an IPI handler context on the remote CPU for
> remote wakeups. This commit appeared upstream in Linux v3.0.
> 
> Unfortunately, ttwu_do_wakeup() happens to contain the "sched_wakeup"
> tracepoint. Analyzing wakup latencies depends on getting the wakeup
> chain right: which process is the waker, which is the wakee. Moving this
> instrumention outside of the waker context prevents trace analysis tools
> from getting the waker pid, either through "current" in the tracepoint
> probe, or by deducing it using other scheduler events based on the CPU
> executing the tracepoint.
> 
> Another side-effect of moving this instrumentation to the scheduler ipi
> is that the delay during which the wakeup is sitting in the pending
> queue is not accounted for when calculating wakeup latency.
> 
> Therefore, move the sched_wakeup instrumentation back to the waker
> context to fix those two shortcomings.

What do you consider wakeup-latency? I don't see how moving the
tracepoint into the caller will magically account the queue time.

> +++ b/kernel/sched/core.c
> @@ -1457,7 +1457,6 @@ static void
>  ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
>  {
>  	check_preempt_curr(rq, p, wake_flags);
> -	trace_sched_wakeup(p, true);
>  
>  	p->state = TASK_RUNNING;
>  #ifdef CONFIG_SMP
> @@ -1505,6 +1504,7 @@ static int ttwu_remote(struct task_struct *p, int wake_flags)
>  	if (task_on_rq_queued(p)) {
>  		/* check_preempt_curr() may use rq clock */
>  		update_rq_clock(rq);
> +		trace_sched_wakeup(p, true);
>  		ttwu_do_wakeup(rq, p, wake_flags);
>  		ret = 1;
>  	}
> @@ -1619,6 +1619,7 @@ static void ttwu_queue(struct task_struct *p, int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
>  
> +	trace_sched_wakeup(p, true);
>  #if defined(CONFIG_SMP)
>  	if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
>  		sched_clock_cpu(cpu); /* sync clocks x-cpu */

You only need one site in try_to_wake_up(), put it right after
success=1.
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