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

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Message-ID: <Y0FwC1yo1pcyL9J/@hirez.programming.kicks-ass.net>
Date:   Sat, 8 Oct 2022 14:41:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
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 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.