From: "Jin, Yao" <yao.jin@linux.intel.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: acme@kernel.org, jolsa@kernel.org, peterz@infradead.org,
mingo@redhat.com, alexander.shishkin@linux.intel.com,
Linux-kernel@vger.kernel.org, ak@linux.intel.com,
kan.liang@intel.com, yao.jin@intel.com
Subject: Re: [PATCH] perf evsel: Get group fd from CPU0 for system wide event
Date: Fri, 15 May 2020 14:04:57 +0800 [thread overview]
Message-ID: <68e53765-6f45-9483-7543-0a2f961cdc62@linux.intel.com> (raw)
In-Reply-To: <3e813227-4954-0d4b-bc7a-ca272b18454a@linux.intel.com>
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@lkp-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
next prev parent reply other threads:[~2020-05-15 6:05 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 1:34 [PATCH] perf evsel: Get group fd from CPU0 for system wide event Jin Yao
2020-05-01 10:23 ` Jiri Olsa
2020-05-02 2:33 ` Jin, Yao
2020-05-05 0:03 ` Jiri Olsa
2020-05-09 7:37 ` Jin, Yao
2020-05-15 6:04 ` Jin, Yao [this message]
2020-05-15 8:33 ` Jiri Olsa
2020-05-18 3:28 ` Jin, Yao
2020-05-20 5:36 ` Jin, Yao
2020-05-20 7:50 ` Jiri Olsa
2020-05-21 4:38 ` Jin, Yao
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