Re: [PATCH] Revert "perf/x86: Allow zero PEBS status with only single active event"

["Liang, Kan" <kan.liang@linux.intel.com>]

On 3/16/2021 2:34 PM, Stephane Eranian wrote:
> On Tue, Mar 16, 2021 at 5:28 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>
>>
>>
>> On 3/16/2021 3:22 AM, Namhyung Kim wrote:
>>> Hi Peter and Kan,
>>>
>>> On Thu, Mar 4, 2021 at 5:22 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>>>
>>>> On Wed, Mar 03, 2021 at 02:53:00PM -0500, Liang, Kan wrote:
>>>>> On 3/3/2021 1:59 PM, Peter Zijlstra wrote:
>>>>>> On Wed, Mar 03, 2021 at 05:42:18AM -0800, kan.liang@linux.intel.com wrote:
>>>>
>>>>>>> +++ b/arch/x86/events/intel/ds.c
>>>>>>> @@ -2000,18 +2000,6 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
>>>>>>>                             continue;
>>>>>>>                     }
>>>>>>> -         /*
>>>>>>> -          * On some CPUs the PEBS status can be zero when PEBS is
>>>>>>> -          * racing with clearing of GLOBAL_STATUS.
>>>>>>> -          *
>>>>>>> -          * Normally we would drop that record, but in the
>>>>>>> -          * case when there is only a single active PEBS event
>>>>>>> -          * we can assume it's for that event.
>>>>>>> -          */
>>>>>>> -         if (!pebs_status && cpuc->pebs_enabled &&
>>>>>>> -                 !(cpuc->pebs_enabled & (cpuc->pebs_enabled-1)))
>>>>>>> -                 pebs_status = cpuc->pebs_enabled;
>>>>>>
>>>>>> Wouldn't something like:
>>>>>>
>>>>>>                       pebs_status = p->status = cpus->pebs_enabled;
>>>>>>
>>>>>
>>>>> I didn't consider it as a potential solution in this patch because I don't
>>>>> think it's a proper way that SW modifies the buffer, which is supposed to be
>>>>> manipulated by the HW.
>>>>
>>>> Right, but then HW was supposed to write sane values and it doesn't do
>>>> that either ;-)
>>>>
>>>>> It's just a personal preference. I don't see any issue here. We may try it.
>>>>
>>>> So I mostly agree with you, but I think it's a shame to unsupport such
>>>> chips, HSW is still a plenty useable chip today.
>>>
>>> I got a similar issue on ivybridge machines which caused kernel crash.
>>> My case it's related to the branch stack with PEBS events but I think
>>> it's the same issue.  And I can confirm that the above approach of
>>> updating p->status fixed the problem.
>>>
>>> I've talked to Stephane about this, and he wants to make it more
>>> robust when we see stale (or invalid) PEBS records.  I'll send the
>>> patch soon.
>>>
>>
>> Hi Namhyung,
>>
>> In case you didn't see it, I've already submitted a patch to fix the
>> issue last Friday.
>> https://lore.kernel.org/lkml/1615555298-140216-1-git-send-email-kan.liang@linux.intel.com/
>> But if you have a more robust proposal, please feel free to submit it.
>>
> This fixes the problem on the older systems. The other problem we
> identified related to the
> PEBS sample processing code is that you can end up with uninitialized
> perf_sample_data
> struct passed to perf_event_overflow():
> 
>   setup_pebs_fixed_sample_data(pebs, data)
> {
>          if (!pebs)
>                  return;
>          perf_sample_data_init(data);  <<< must be moved before the if (!pebs)
>          ...
> }
> 
> __intel_pmu_pebs_event(pebs, data)
> {
>          setup_sample(pebs, data)
>          perf_event_overflow(data);
>          ...
> }
> 
> If there is any other reason to get a pebs = NULL in fix_sample_data()
> or adaptive_sample_data(), then
> you must call perf_sample_data_init(data) BEFORE you return otherwise
> you end up in perf_event_overflow()
> with uninitialized data and you may die as follows:
> 
> [<ffffffff812f283d>] ? perf_output_copy+0x4d/0xb0
> [<ffffffff812e0fb1>] perf_output_sample+0x561/0xab0
> [<ffffffff812e0952>] ? __perf_event_header__init_id+0x112/0x130
> [<ffffffff812e1be1>] ? perf_prepare_sample+0x1b1/0x730
> [<ffffffff812e21b9>] perf_event_output_forward+0x59/0x80
> [<ffffffff812e0634>] ? perf_event_update_userpage+0xf4/0x110
> [<ffffffff812e4468>] perf_event_overflow+0x88/0xe0
> [<ffffffff810175b8>] __intel_pmu_pebs_event+0x328/0x380
> 
> This all stems from get_next_pebs_record_by_bit()  potentially
> returning NULL but the NULL is not handled correctly
> by the callers. This is what I'd like to see cleaned up in
> __intel_pmu_pebs_event() to  avoid future problems.
> 

Got it. Yes, we need another patch to correctly handle the potentially
returning NULL. Thanks for pointing it out.

Thanks,
Kan