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Date: Fri, 15 May 2020 14:04:57 +0800 From: "Jin, Yao" <yao.jin@...ux.intel.com> To: Jiri Olsa <jolsa@...hat.com> Cc: acme@...nel.org, jolsa@...nel.org, peterz@...radead.org, mingo@...hat.com, alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org, ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com Subject: Re: [PATCH] perf evsel: Get group fd from CPU0 for system wide event Hi Jiri, On 5/9/2020 3:37 PM, Jin, Yao wrote: > Hi Jiri, > > On 5/5/2020 8:03 AM, Jiri Olsa wrote: >> On Sat, May 02, 2020 at 10:33:59AM +0800, Jin, Yao wrote: >> >> SNIP >> >>>>> @@ -1461,6 +1461,9 @@ static int get_group_fd(struct evsel *evsel, int cpu, int thread) >>>>> BUG_ON(!leader->core.fd); >>>>> fd = FD(leader, cpu, thread); >>>>> + if (fd == -1 && leader->core.system_wide) >>>> >>>> fd does not need to be -1 in here.. in my setup cstate_pkg/c2-residency/ >>>> has cpumask 0, so other cpus never get open and are 0, and the whole thing >>>> ends up with: >>>> >>>> sys_perf_event_open: pid -1 cpu 1 group_fd 0 flags 0 >>>> sys_perf_event_open failed, error -9 >>>> >>>> I actualy thought we put -1 to fd array but couldn't find it.. perhaps we should od that >>>> >>>> >>> >>> I have tested on two platforms. On KBL desktop fd is 0 for this case, but on >>> oncascadelakex server, fd is -1, so the BUG_ON(fd == -1) is triggered. >>> >>>>> + fd = FD(leader, 0, thread); >>>>> + >>>> >>>> so how do we group following events? >>>> >>>> cstate_pkg/c2-residency/ - cpumask 0 >>>> msr/tsc/ - all cpus >>>> >>> >>> Not sure if it's enough to only use cpumask 0 because >>> cstate_pkg/c2-residency/ should be per-socket. >>> >>>> cpu 0 is fine.. the rest I have no idea ;-) >>>> >>> >>> Perhaps we directly remove the BUG_ON(fd == -1) assertion? >> >> I think we need to make clear how to deal with grouping over >> events that comes for different cpus >> >> so how do we group following events? >> >> cstate_pkg/c2-residency/ - cpumask 0 >> msr/tsc/ - all cpus >> >> >> what's the reason/expected output of groups with above events? >> seems to make sense only if we limit msr/tsc/ to cpumask 0 as well >> >> jirka >> > > On 2-socket machine (e.g cascadelakex), "cstate_pkg/c2-residency/" is per-socket event and the > cpumask is 0 and 24. > > root@...-csl-2sp5 /sys/devices/cstate_pkg# cat cpumask > 0,24 > > We can't limit it to cpumask 0. It should be programmed on CPU0 and CPU24 (the first CPU on each > socket). > > The "msr/tsc" are per-cpu event, it should be programmed on all cpus. So I don't think we can limit > msr/tsc to cpumask 0. > > The issue is how we deal with get_group_fd(). > > static int get_group_fd(struct evsel *evsel, int cpu, int thread) > { > struct evsel *leader = evsel->leader; > int fd; > > if (evsel__is_group_leader(evsel)) > return -1; > > /* > * Leader must be already processed/open, > * if not it's a bug. > */ > BUG_ON(!leader->core.fd); > > fd = FD(leader, cpu, thread); > BUG_ON(fd == -1); > > return fd; > } > > When evsel is "msr/tsc/", > > FD(leader, 0, 0) is 3 (3 is the fd of "cstate_pkg/c2-residency/" on CPU0) > FD(leader, 1, 0) is -1 > BUG_ON asserted. > > If we just return group_fd(-1) for "msr/tsc", it looks like it's not a problem, is it? > > Thanks > Jin Yao 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; + /* * 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
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