Re: [PATCH v2] watchdog: don't run proc_watchdog_update if new value is same as old

[Aaron Tomlin <atomlin@redhat.com>]

On Tue 2016-03-15 00:19 -0400, 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().
> 
> [  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.
> 
> There doesn't appear to be a reason for doing this work every time a write
> occurs, so only do it when the values change.
> 
> Signed-off-by: Josh Hunt <johunt@akamai.com>
> Cc: <stable@vger.kernel.org> # 4.1.x-
> ---
>  kernel/watchdog.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> v2: Update changelog to no longer include mention of soft lockup, and add stable.
> 
> 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) {

Reviewed-by: Aaron Tomlin <atomlin@redhat.com>