Messages in this thread | | | Date | Tue, 20 Aug 2019 16:10:14 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] perf/x86: Consider pinned events for group validation |
| |
On Fri, Aug 16, 2019 at 10:49:10AM -0700, kan.liang@linux.intel.com wrote: > From: Kan Liang <kan.liang@linux.intel.com> > > perf stat -M metrics relies on weak groups to reject unschedulable > groups and run them as non-groups. > This uses the group validation code in the kernel. Unfortunately > that code doesn't take pinned events, such as the NMI watchdog, into > account. So some groups can pass validation, but then later still > never schedule.
But if you first create the group and then a pinned event it 'works', which is inconsistent and makes all this timing dependent.
> @@ -2011,9 +2011,11 @@ static int validate_event(struct perf_event *event) > */ > static int validate_group(struct perf_event *event) > { > + struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events); > struct perf_event *leader = event->group_leader; > struct cpu_hw_events *fake_cpuc; > - int ret = -EINVAL, n; > + struct perf_event *pinned_event; > + int ret = -EINVAL, n, i; > > fake_cpuc = allocate_fake_cpuc(); > if (IS_ERR(fake_cpuc)) > @@ -2033,6 +2035,24 @@ static int validate_group(struct perf_event *event) > if (n < 0) > goto out; > > + /* > + * The new group must can be scheduled > + * together with current pinned events. > + * Otherwise, it will never get a chance > + * to be scheduled later.
That's wrapped short; also I don't think it is sufficient; what if you happen to have a pinned event on CPU1 (and not others) and happen to run validation for a new CPU1 event on CPUn ?
Also; per that same; it is broken, you're accessing the cpu-local cpuc without serialization.
> + */ > + for (i = 0; i < cpuc->n_events; i++) { > + pinned_event = cpuc->event_list[i]; > + if (WARN_ON_ONCE(!pinned_event)) > + continue; > + if (!pinned_event->attr.pinned) > + continue; > + fake_cpuc->n_events = n; > + n = collect_events(fake_cpuc, pinned_event, false); > + if (n < 0) > + goto out; > + } > + > fake_cpuc->n_events = 0; > ret = x86_pmu.schedule_events(fake_cpuc, n, NULL); > > -- > 2.7.4 >
| |