Re: [RFC PATCH] sched: Fix sched_wakeup tracepoint

Peter Zijlstra
2015-06-05T12:47:00Z

On Fri, 2015-06-05 at 14:32 +0200, Thomas Gleixner wrote:
> On Fri, 5 Jun 2015, Peter Zijlstra wrote:
> > 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.
>
> Well, the point of wakeup is when the wakee calls wakeup. If the trace
> point is in the IPI then you account the time between the wakeup and
> the actuall handling in the IPI to the wakee instead of accounting it
> to the time between wakeup and sched switch.

My point exactly, wake->schedule is what we call the scheduling latency,
not the wake latency, which would be from 'event' to the task being
runnable.