On Tue, Oct 08, 2019 at 09:59:49AM -0700, Song Liu wrote:
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).
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 cycles 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%)
To fix this, when the flexible_active list is empty, try rotate the first event in the flexible_groups. Also, rename ctx_first_active() to ctx_event_to_rotate(), which is more accurate.
Fixes: 8d5bce0c37fa ("perf/core: Optimize perf_rotate_context() event scheduling") Cc: stable@vger.kernel.org # v4.17+ Cc: Peter Zijlstra peterz@infradead.org Cc: Arnaldo Carvalho de Melo acme@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Sasha Levin sashal@kernel.org Signed-off-by: Song Liu songliubraving@fb.com
Thanks!