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Date: Tue, 31 Oct 2017 15:11:07 -0700
From: Guenter Roeck <linux@roeck-us.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Don Zickus <dzickus@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: Crashes in perf_event_ctx_lock_nested
Message-ID: <20171031221107.GA12133@roeck-us.net>
References: <20171030224512.GA13592@roeck-us.net>
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On Tue, Oct 31, 2017 at 10:32:00PM +0100, Thomas Gleixner wrote:

[ ...] 

> So we have to revert
> 
> a33d44843d45 ("watchdog/hardlockup/perf: Simplify deferred event destroy")
> 
> Patch attached.
> 

Tested-by: Guenter Roeck <linux@roeck-us.net>

There is still a problem. When running

echo 6 > /proc/sys/kernel/watchdog_thresh
echo 5 > /proc/sys/kernel/watchdog_thresh

repeatedly, the message

NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.

stops after a while (after ~10-30 iterations, with fluctuations).
After adding trace messages into hardlockup_detector_perf_disable()
and hardlockup_detector_perf_enable(), I see:

hardlockup_detector_perf_disable: disable(0): Number of CPUs: 3
hardlockup_detector_perf_disable: disable(1): Number of CPUs: 2
hardlockup_detector_perf_disable: disable(2): Number of CPUs: 1
hardlockup_detector_perf_disable: disable(3): Number of CPUs: 0
...
hardlockup_detector_perf_disable: disable(0): Number of CPUs: 2
hardlockup_detector_perf_disable: disable(1): Number of CPUs: 1
hardlockup_detector_perf_disable: disable(2): Number of CPUs: 0
hardlockup_detector_perf_disable: disable(3): Number of CPUs: -1
...
hardlockup_detector_perf_enable: enable(1): Number of CPUs: -6
hardlockup_detector_perf_enable: enable(3): Number of CPUs: -5
hardlockup_detector_perf_enable: enable(2): Number of CPUs: -4
hardlockup_detector_perf_enable: enable(0): Number of CPUs: -3

Maybe watchdog_cpus needs to be atomic ?

Guenter