Re: [RFC PATCH] sched: Fix sched_wakeup tracepoint

Peter Zijlstra
2015-06-05T12:09:16Z

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.