linux-kernel - Re: [PATCH] perf: Fix missing SIGTRAPs

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Date:   Mon, 10 Oct 2022 22:52:08 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] perf: Fix missing SIGTRAPs

On Sat, Oct 08, 2022 at 02:41PM +0200, Peter Zijlstra wrote:
> On Sat, Oct 08, 2022 at 10:41:28AM +0200, Marco Elver wrote:
> > The below patch to the sigtrap_threads test can repro the issue (when
> > run lots of them concurrently again). It also illustrates the original
> > problem we're trying to solve, where the event never gets rearmed again
> > and the test times out (doesn't happen with the almost-working fix).
> 
> Excellent, that helps. Also, I'm an idiot ;-)
> 
> The below seems to fix it for me.
> 
> ---
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3441,7 +3448,8 @@ static void perf_event_context_sched_out
>  			perf_pmu_disable(pmu);
>  
>  			/* PMIs are disabled; ctx->nr_pending is stable. */
> -			if (local_read(&ctx->nr_pending)) {
> +			if (local_read(&ctx->nr_pending) ||
> +			    local_read(&next_ctx->nr_pending)) {
>  				/*
>  				 * Must not swap out ctx when there's pending
>  				 * events that rely on the ctx->task relation.

Yup, that fixes it.

Can you send a v2 with all the fixups? Just to make sure I've tested the
right thing.

I'll also send the patch for the selftest addition once I gave it a good
spin.

Thanks,
-- Marco