linux-kernel - Re: [PATCH 3/3] softlockup: fix watchdog task wakeup frequency

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Message-ID: <87prq3xdjv.fsf@skyscraper.fehenstaub.lan>
Date:	Fri, 27 Jun 2008 14:33:40 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 3/3] softlockup: fix watchdog task wakeup frequency

Ingo Molnar <mingo@...e.hu> writes:

> * Johannes Weiner <hannes@...urebad.de> wrote:
>
>> Wake up the watchdog every second instead of every second. [...]
>
> now _that_ is a real improvement ;-)
>
>>  	/* Wake up the high-prio watchdog task every second: */
>> -	if (now > (touch_timestamp + 1))
>> +	if (now > touch_timestamp)
>>  		wake_up_process(per_cpu(watchdog_task, this_cpu));
>
> you mean wake up every second instead of every two seconds?

Every second second :D The second `second' [adjective] refers to the
first `second' [noun].

> i think the best sleep period for the watchdog is half of the threshold. 
> It makes no point to check sooner than that. I.e. softlockup_thresh/2 
> would be better?

Hm, it updates the timestamp, so it makes only sense if it runs at a
maximum every second (timestamp granularity) or even less.  The check
for hung tasks uses the cpu timestamp as well for comparison, so that
would be okay too.

Like this?

diff --git a/kernel/softlockup.c b/kernel/softlockup.c
index c828c23..b884546 100644
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -106,8 +106,9 @@ void softlockup_tick(void)
 
 	now = get_timestamp(this_cpu);
 
-	/* Wake up the high-prio watchdog task every second: */
-	if (now > (touch_timestamp + 1))
+	/* Wake up the high-prio watchdog task twice per
+	 * threshold timespan. */
+	if (now > (touch_timestamp + softlockup_thresh / 2))
 		wake_up_process(per_cpu(watchdog_task, this_cpu));
 
 	/* Warn about unreasonable delays: */
--
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