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

From: Liang, Kan
Date: Tue Mar 16 2021 - 08:29:38 EST

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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx 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
- /*
- * 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.
But if you have a more robust proposal, please feel free to submit it.

BTW: The patch set from last Friday also fixed another bug found by the perf_fuzzer test. You may be interested.