Date: Mon, 14 Mar 2016 10:34:26 -0400
From: Don Zickus <dzickus@redhat.com>
To: Joshua Hunt <johunt@akamai.com>
Cc: akpm@linux-foundation.org, uobergfe@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] watchdog: don't run proc_watchdog_update if new value is
 same as old
Message-ID: <20160314143426.GK194535@redhat.com>
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On Sat, Mar 12, 2016 at 06:50:26PM -0500, Joshua Hunt wrote:
> While working on a script to restore all sysctl params before a series of
> tests I found that writing any value into the
> /proc/sys/kernel/{nmi_watchdog,soft_watchdog,watchdog,watchdog_thresh}
> causes them to call proc_watchdog_update(). Not only that, but when I
> wrote to these proc files in a loop I could easily trigger a soft lockup.
> 
> [  955.756196] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> [  955.765994] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> [  955.774619] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> [  955.783182] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> [  959.788319] NMI watchdog: BUG: soft lockup - CPU#4 stuck for 30s! [swapper/4:0]
> [  959.788325] NMI watchdog: BUG: soft lockup - CPU#5 stuck for 30s! [swapper/5:0]
> 
> There doesn't appear to be a reason for doing this work other every time a
> write occurs, so only do the work when the values change.

Hi Josh,

Thanks for the patch.  I have no objections to it, but Uli and myself were
interested in the reason for the softlockups.  Uli is going to provide a
test patch to see if his theory is correct.  That way we fix the underlying
issue and then apply your patch on top. Make sense?

Cheers,
Don

> 
> Signed-off-by: Josh Hunt <johunt@akamai.com>
> ---
>  kernel/watchdog.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index b3ace6e..9acb29f 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -923,6 +923,9 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
>  		 * both lockup detectors are disabled if proc_watchdog_update()
>  		 * returns an error.
>  		 */
> +		if (old == new)
> +			goto out;
> +
>  		err = proc_watchdog_update();
>  	}
>  out:
> @@ -967,7 +970,7 @@ int proc_soft_watchdog(struct ctl_table *table, int write,
>  int proc_watchdog_thresh(struct ctl_table *table, int write,
>  			 void __user *buffer, size_t *lenp, loff_t *ppos)
>  {
> -	int err, old;
> +	int err, old, new;
>  
>  	get_online_cpus();
>  	mutex_lock(&watchdog_proc_mutex);
> @@ -987,6 +990,10 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
>  	/*
>  	 * Update the sample period. Restore on failure.
>  	 */
> +	new = ACCESS_ONCE(watchdog_thresh);
> +	if (old == new)
> +		goto out;
> +
>  	set_sample_period();
>  	err = proc_watchdog_update();
>  	if (err) {
> -- 
> 1.7.9.5
>