Re: [PATCH V9 3/3] perf/x86/intel: Support PEBS counters snapshotting

Liang, Kan
2025-01-16T21:50:04Z

On 2025-01-16 3:56 p.m., Peter Zijlstra wrote:
> On Thu, Jan 16, 2025 at 09:42:25PM +0100, Peter Zijlstra wrote:
>> On Thu, Jan 16, 2025 at 10:55:46AM -0500, Liang, Kan wrote:
>>
>>>> Also, I think I found you another bug... Consider what happens to the
>>>> counter value when we reschedule a HES_STOPPED counter, then we skip
>>>> x86_pmu_start(RELOAD) on step2, which leave the counter value with
>>>> 'random' crap from whatever was there last.
>>>>
>>>> But meanwhile you do program PEBS to sample it. That will happily sample
>>>> this garbage.
>>>>
>>>> Hmm?
>>>
>>> I'm not quite sure I understand the issue.
>>>
>>> The HES_STOPPED counter should be a pre-existing counter. Just for some
>>> reason, it's stopped, right? So perf doesn't need to re-configure the
>>> PEBS__DATA_CFG, since the idx is not changed.
>>
>> Suppose you have your group {A, B, C} and lets suppose A is the PEBS
>> event, further suppose that B is also a sampling event. Lets say they
>> get hardware counters 1,2 and 3 respectively.
>>
>> Then lets say B gets throttled.
>>
>> While it is throttled, we get a new event D scheduled, and D gets placed
>> on counter 2 -- where B lives, which gets moved over to counter 4.
>>
>> Then our loops will update and remove B from 2, but because
>> throttled/HES_STOPPED it will not start it on counter 4.
>>>> Meanwhile, we do have the PEBS_DATA_CFG thing updated to sample counter
>> 1,3 and 4.
>>
>> PEBS assist happens, and samples the uninitialized counter 4.
> > Also, by skipping x86_pmu_start() we miss the assignment of
> cpuc->events[] so PEBS buffer decode can't even find the dodgy event.
>

Yes, counter 4 includes garbage before the B is started again.
But the cpuc->events[counter 4] is NULL either.

The current implementation ignores the NULL cpuc->events[]. The stopped
B should not be mistakenly updated.

+static void intel_perf_event_pmc_to_count(struct perf_event *event, u64
pmc)
+{
+ int shift = 64 - x86_pmu.cntval_bits;
+ struct hw_perf_event *hwc;
+ u64 delta, prev_pmc;
+
+ /*
+ * The PEBS record doesn't shrink on pmu::del().
+ * See pebs_update_state().
+ * Ignore the non-exist event.
+ */
+ if (!event)
+ return;

Thanks,
Kan