On Mon, Aug 07, 2017 at 02:15:51PM +0200, Peter Zijlstra wrote: > On Fri, Aug 04, 2017 at 04:50:54PM +0800, 石祤 wrote: > > From: "leilei.lin" <[email protected]> > > > > A performance issue caused by less strickly check in task > > sched when these tasks were once attached by per-task perf_event. > > > > A task will alloc task->perf_event_ctxp[ctxn] when it was called > > by perf_event_open, and task->perf_event_ctxp[ctxn] would not > > ever be freed to NULL. > > > > __perf_event_task_sched_in() > > if (task->perf_event_ctxp[ctxn]) // here is always true > > perf_event_context_sched_in() // operate pmu > > > > 50% at most performance overhead was observed under some extreme > > test case. Therefor, add a more strick check as to ctx->nr_events, > > when ctx->nr_events == 0, it's no need to continue. > > > > Signed-off-by: leilei.lin <[email protected]> > > --- > > kernel/events/core.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/kernel/events/core.c b/kernel/events/core.c > > index 426c2ff..f071013 100644 > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -3179,6 +3179,9 @@ static void perf_event_context_sched_in(struct > > perf_event_context *ctx, > > if (cpuctx->task_ctx == ctx) > > return; > > > > + if (!cpuctx->task_ctx && !ctx->nr_events) > > + return; > > + > > perf_ctx_lock(cpuctx, ctx); > > perf_pmu_disable(ctx->pmu); > > /* > > I _think_ we must do this check after acquiring ctx->lock, because of > how perf_install_in_context() works. See commit: > > 63cae12bce98 ("perf/core: Fix sys_perf_event_open() vs. hotplug") > > That is a giant bag of tricky, but the gist of it is that if > perf_event_ctxp[] is !NULL (as is the case in your scenario) then we > must acquire ctx->lock.
The alternative is of course, trying to free the task ctx. I've not gone through what would be required to make that happen.
