Re: [PATCH] perf/core: fix corner case in perf_rotate_context()

[Song Liu <songliubraving@fb.com>]

> On Oct 2, 2019, at 11:43 PM, Song Liu <songliubraving@fb.com> wrote:
> 
> This is a rare corner case, but it does happen:
> 
> In perf_rotate_context(), when the first cpu flexible event fail to
> schedule, cpu_rotate is 1, while cpu_event is NULL. Since cpu_event is
> NULL, perf_rotate_context will _NOT_ call cpu_ctx_sched_out(), thus
> cpuctx->ctx.is_active will have EVENT_FLEXIBLE set. Then, the next
> perf_event_sched_in() will skip all cpu flexible events because of the
> EVENT_FLEXIBLE bit.
> 
> In the next call of perf_rotate_context(), cpu_rotate stays 1, and
> cpu_event stays NULL, so this process repeats. The end result is, flexible
> events on this cpu will not be scheduled (until another event being added
> to the cpuctx).
> 
> Similar issue may happen with the task_ctx. But it is usually not a
> problem because the task_ctx moves around different CPU.
> 
> Fix this corner case by using cpu_rotate and task_rotate to gate calls for
> (cpu_)ctx_sched_out and rotate_ctx. Also enable rotate_ctx() to handle
> event == NULL case.

Here is an easy repro of this issue. On Intel CPUs, where ref-cycles 
could only use one counter, run one pinned event for ref-cycles, one
flexible event for ref-cycles, and one flexible event for cycles. The 
flexible ref-cycles is never scheduled, which is expected. However, 
because of this issue, the cycle event is never scheduled either. 

perf stat -e ref-cycles:D,ref-cycles,cycles -C 5 -I 1000
#           time             counts unit events
     1.000152973         15,412,480      ref-cycles:D
     1.000152973      <not counted>      ref-cycles     (0.00%)
     1.000152973      <not counted>      cycles         (0.00%)
     2.000486957         18,263,120      ref-cycles:D
     2.000486957      <not counted>      ref-cycles     (0.00%)
     2.000486957      <not counted>      cycles         (0.00%)

Thanks,
Song