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

Josh Hunt
2016-03-14T14:45:33Z

On 03/14/2016 09:34 AM, Don Zickus wrote:
> 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?

Yep. Sounds good. I meant to mention I didn't diagnose the soft-lockup.
If you provide a patch I'm happy to test. I can also attempt to debug
that part more if needed.

Josh

>
> 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
>>