linux-kernel - Re: [PATCH V3] perf: Fix the throttle error of some clock events

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Message-ID: <CAADnVQKjyzdNVR_+WCMzORPJAX00tD3HK0vaCz13ZprWaG72Tg@mail.gmail.com>
Date: Wed, 4 Jun 2025 16:21:50 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Kan Liang <kan.liang@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, 
	Mark Rutland <mark.rutland@....com>, LKML <linux-kernel@...r.kernel.org>, 
	"linux-perf-use." <linux-perf-users@...r.kernel.org>, Stephane Eranian <eranian@...gle.com>, 
	Chun-Tse Shao <ctshao@...gle.com>, Thomas Richter <tmricht@...ux.ibm.com>, Leo Yan <leo.yan@....com>, 
	Aishwarya TCV <aishwarya.tcv@....com>, Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Subject: Re: [PATCH V3] perf: Fix the throttle error of some clock events

On Wed, Jun 4, 2025 at 10:16 AM <kan.liang@...ux.intel.com> wrote:
>
> From: Kan Liang <kan.liang@...ux.intel.com>
>
> Both ARM and IBM CI reports RCU stall, which can be reproduced by the
> below perf command.
>   perf record -a -e cpu-clock -- sleep 2
>
> The issue is introduced by the generic throttle patch set, which
> unconditionally invoke the event_stop() when throttle is triggered.
>
> The cpu-clock and task-clock are two special SW events, which rely on
> the hrtimer. The throttle is invoked in the hrtimer handler. The
> event_stop()->hrtimer_cancel() waits for the handler to finish, which is
> a deadlock. Instead of invoking the stop(), the HRTIMER_NORESTART should
> be used to stop the timer.
>
> There may be two ways to fix it.
> - Introduce a PMU flag to track the case. Avoid the event_stop in
>   perf_event_throttle() if the flag is detected.
>   It has been implemented in the
>   https://lore.kernel.org/lkml/20250528175832.2999139-1-kan.liang@linux.intel.com/
>   The new flag was thought to be an overkill for the issue.
> - Add a check in the event_stop. Return immediately if the throttle is
>   invoked in the hrtimer handler. Rely on the existing HRTIMER_NORESTART
>   method to stop the timer.
>
> The latter is implemented here.
>
> Move event->hw.interrupts = MAX_INTERRUPTS before the stop(). It makes
> the order the same as perf_event_unthrottle(). Except the patch, no one
> checks the hw.interrupts in the stop(). There is no impact from the
> order change.
>
> Reported-by: Leo Yan <leo.yan@....com>
> Reported-by: Aishwarya TCV <aishwarya.tcv@....com>
> Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@e132581.arm.com/
> Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Closes: https://lore.kernel.org/lkml/djxlh5fx326gcenwrr52ry3pk4wxmugu4jccdjysza7tlc5fef@ktp4rffawgcw/
> Reported-by: Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
> Closes: https://lore.kernel.org/lkml/8e8f51d8-af64-4d9e-934b-c0ee9f131293@linux.ibm.com/
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>

It seems the patch fixes one issue and introduces another ?

Looks like the throttle event is sticky.
Once it's reached the perf_event no longer works ?
Repro:
test_progs -t perf_branches/perf_branches_no_hw
#250/2   perf_branches/perf_branches_no_hw:OK

test_progs -t stacktrace_build_id_nmi
#393     stacktrace_build_id_nmi:OK

test_progs -t perf_branches/perf_branches_no_hw
perf_branches/perf_branches_no_hw:FAIL

Maybe it's an unrelated bug.