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

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Message-ID: <20250116205659.GA15641@noisy.programming.kicks-ass.net>
Date: Thu, 16 Jan 2025 21:56:59 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	ak@...ux.intel.com, eranian@...gle.com, dapeng1.mi@...ux.intel.com
Subject: Re: [PATCH V9 3/3] perf/x86/intel: Support PEBS counters snapshotting

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.