Messages in this thread | | | Subject | Re: [PATCH] perf evsel: Get group fd from CPU0 for system wide event | From | "Jin, Yao" <> | Date | Thu, 21 May 2020 12:38:08 +0800 |
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Hi Jiri,
On 5/20/2020 3:50 PM, Jiri Olsa wrote: > On Wed, May 20, 2020 at 01:36:40PM +0800, Jin, Yao wrote: >> Hi Jiri, >> >> On 5/18/2020 11:28 AM, Jin, Yao wrote: >>> Hi Jiri, >>> >>> On 5/15/2020 4:33 PM, Jiri Olsa wrote: >>>> On Fri, May 15, 2020 at 02:04:57PM +0800, Jin, Yao wrote: >>>> >>>> SNIP >>>> >>>>> I think I get the root cause. That should be a serious bug in get_group_fd, access violation! >>>>> >>>>> For a group mixed with system-wide event and per-core event and the group >>>>> leader is system-wide event, access violation will happen. >>>>> >>>>> perf_evsel__alloc_fd allocates one FD member for system-wide event (only FD(evsel, 0, 0) is valid). >>>>> >>>>> But for per core event, perf_evsel__alloc_fd allocates N FD members (N = >>>>> ncpus). For example, for ncpus is 8, FD(evsel, 0, 0) to FD(evsel, 7, 0) are >>>>> valid. >>>>> >>>>> get_group_fd(struct evsel *evsel, int cpu, int thread) >>>>> { >>>>> struct evsel *leader = evsel->leader; >>>>> >>>>> fd = FD(leader, cpu, thread); /* access violation may happen here */ >>>>> } >>>>> >>>>> If leader is system-wide event, only the FD(leader, 0, 0) is valid. >>>>> >>>>> When get_group_fd accesses FD(leader, 1, 0), access violation happens. >>>>> >>>>> My fix is: >>>>> >>>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c >>>>> index 28683b0eb738..db05b8a1e1a8 100644 >>>>> --- a/tools/perf/util/evsel.c >>>>> +++ b/tools/perf/util/evsel.c >>>>> @@ -1440,6 +1440,9 @@ static int get_group_fd(struct evsel *evsel, int cpu, int thread) >>>>> if (evsel__is_group_leader(evsel)) >>>>> return -1; >>>>> >>>>> + if (leader->core.system_wide && !evsel->core.system_wide) >>>>> + return -2; >>>> >>>> so this effectively stops grouping system_wide events with others, >>>> and I think it's correct, how about events that differ in cpumask? >>>> >>> >>> My understanding for the events that differ in cpumaks is, if the >>> leader's cpumask is not fully matched with the evsel's cpumask then we >>> stop the grouping. Is this understanding correct? >>> >>> I have done some tests and get some conclusions: >>> >>> 1. If the group is mixed with core and uncore events, the system_wide checking can distinguish them. >>> >>> 2. If the group is mixed with core and uncore events and "-a" is >>> specified, the system_wide for core event is also false. So system_wide >>> checking can distinguish them too >>> >>> 3. In my test, the issue only occurs when we collect the metric which is >>> mixed with uncore event and core event, so maybe checking the >>> system_wide is OK. >>> >>>> should we perhaps ensure this before we call open? go throught all >>>> groups and check they are on the same cpus? >>>> >>> >>> The issue doesn't happen at most of the time (only for the metric >>> consisting of uncore event and core event), so fallback to stop grouping >>> if call open is failed looks reasonable. >>> >>> Thanks >>> Jin Yao >>> >>>> thanks, >>>> jirka >>>> >>>> >>>>> + >>>>> /* >>>>> * Leader must be already processed/open, >>>>> * if not it's a bug. >>>>> @@ -1665,6 +1668,11 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus, >>>>> pid = perf_thread_map__pid(threads, thread); >>>>> >>>>> group_fd = get_group_fd(evsel, cpu, thread); >>>>> + if (group_fd == -2) { >>>>> + errno = EINVAL; >>>>> + err = -EINVAL; >>>>> + goto out_close; >>>>> + } >>>>> retry_open: >>>>> test_attr__ready(); >>>>> >>>>> It enables the perf_evlist__reset_weak_group. And in the second_pass (in >>>>> __run_perf_stat), the events will be opened successfully. >>>>> >>>>> I have tested OK for this fix on cascadelakex. >>>>> >>>>> Thanks >>>>> Jin Yao >>>>> >>>> >> >> Is this fix OK? >> >> Another thing is, do you think if we need to rename >> "evsel->core.system_wide" to "evsel->core.has_cpumask". >> >> The "system_wide" may misleading. >> >> evsel->core.system_wide = pmu ? pmu->is_uncore : false; >> >> "pmu->is_uncore" is true if PMU has a "cpumask". But it's not just uncore >> PMU which has cpumask. Some other PMUs, e.g. cstate_pkg, also have cpumask. >> So for this case, "has_cpumask" should be better. > > so those flags are checked in many places in the code so I don't > think it's wise to mess with them > > what I meant before was that the cpumask could be different for > different events so even when both events are 'system_wide' the > leader 'fd' might not exist for the groupped events and vice versa > > so maybe we should ensure that we are groupping events with same > cpu maps before we go for open, so the get_group_fd stays simple >
Thanks for the comments. I'm preparing the patch according to this idea.
>> >> But I'm not sure if the change is OK for other case, e.g. PT, which also >> uses "evsel->core.system_wide". > > plz CC Adrian Hunter <adrian.hunter@intel.com> on next patches > if you are touching this >
I will not touch "evsel->core.system_wide" in the new patch.
Thanks Jin Yao
> thanks, > jirka >
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