Re: [RFC PATCH] sched: Fix sched_wakeup tracepoint

Mathieu Desnoyers
2015-06-05T12:37:06Z

----- On Jun 5, 2015, at 2:32 PM, Thomas Gleixner tglx@linutronix.de 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

^ I think you actually mean "when the waker calls wakeup".

> 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.
>

Thanks,

Mathieu

> Thanks,
>
> tglx

--
Mathieu Desnoyers
EfficiOS Inc.