Re: [perf] WARNING: CPU: 0 PID: 1457 at kernel/events/core.c:890 add_event_to_ctx()

[Mark Rutland]

On Sun, Jan 25, 2015 at 04:34:28AM +0000, Fengguang Wu wrote:
> Greetings,

Hi Fengguang,

Thanks very much for the report.

> 0day kernel testing robot got the below dmesg and the first bad commit is
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git perf/core
> 
> commit d26bb7f73a2881f2412c340a27438b185f0cc3dc
> Author:     Mark Rutland <mark.rutland@xxxxxxx>
> AuthorDate: Wed Jan 7 15:01:54 2015 +0000
> Commit:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CommitDate: Fri Jan 23 15:17:56 2015 +0100
> 
>     perf: decouple unthrottling and rotating

[...]

What seems to be happening is:

* An event is created in the context of task A on CPU0. As the task's hw
  context is empty of events, we call perf_pmu_ctx_activate. This adds
  the cpuctx of the relevant HW PMU to the active_ctx_list. Note that we
  checked the task's ctx for emptiness, then added the PMU's hw context.

* An event is created (as a result of a clone()) in the context of task
  B on CPU0, and we do the same thing, finding the ctx empty of events
  we add the HW PMU's cpuctx to the active_ctx_list. As it's already in
  there, the WARN_ON(!list_empty(&cpuctx->active_ctx_list)) explodes.

So I guess what we actually want to do is to turn the active_ctx list
into a list of perf_event_contexts rather than perf_event_cpu_contexts,
and add/remove from the list when a context is scheduled in/out (or
updated empty<->nonempty). That way we remove chances for erroneous
add/remove, and we don't need to treat task and CPU contexts separately
in perf_event_task_tick.

I managed to get the reproducer running on a box at home, so I'll spin a
potential fix against that for a while, and send that out if I don't see
explosions.

Peter, I guess you'll drop this patch for now?

Thanks,
Mark.
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