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SubjectRe: [PATCH 0/8] perf sched: Add trace event for sched wait.
On Tue, Apr 15, 2014 at 09:32:49PM +0900, Dongsheng Yang wrote:
> Hi all:
> This is to solve the bug message shown in perf sched latency.
>
> # perf sched latency|tail
> ksoftirqd/0:3 | 0.597 ms | 57 | avg: 0.004 ms | max: 0.054 ms | max at: 19681.546204 s
> ksoftirqd/1:14 | 0.637 ms | 58 | avg: 0.004 ms | max: 0.066 ms | max at: 19674.687734 s
> irqbalance:349 | 0.429 ms | 1 | avg: 0.004 ms | max: 0.004 ms | max at: 19675.791528 s
> ksoftirqd/3:24 | 0.527 ms | 67 | avg: 0.003 ms | max: 0.011 ms | max at: 19673.285019 s
> migration/3:23 | 0.000 ms | 1 | avg: 0.002 ms | max: 0.002 ms | max at: 19672.055354 s
> -----------------------------------------------------------------------------------------
> TOTAL: | 4384.616 ms | 36879 |
> ---------------------------------------------------
> INFO: 0.030% state machine bugs (11 out of 36684)
>
> After some investigation, there are two reasons cause this problem.
>
> (1). Sometimes, scheduler will wake up a running task, it is not necessary,
> then I skip the wakeup if task->state is TASK_RUNNING. [4/8]
>
> (2). No tracing for sched wait.
> This is a simple graph for task state changing.
>
> ---------------- 1 ----------------
> | TASK_RUNNING | ------------------------------>| TASK_RUNNING |
> | (running) |<------------------------------ | (wait cpu) |
> ---------------- 2 ----------------
> ^ |
> |4 ------------------------- 3 |
> |-------|TASK_{UN}INTERRUPTABLE |<--------------|
> | in wait_rq |
> -------------------------
>
> As the graph shown above, there are four event in scheduling, and
> we currently are tracing 3 of them.
>
> 1 & 2: sched:sched_switch
> 4: sched:sched_wakeup|sched:sched_wakeup_new
>
> But about 3, we have no trace event for it.

We do, sched_switch() includes the previous task state. Also there's
tons more than a waitqueue you can get stuck in.


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