linux-kernel - Re: PATCH: softlockup: Fix hung_task_check

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Message-ID: <20091130191307.GM11670@google.com>
Date:	Mon, 30 Nov 2009 11:13:07 -0800
From:	Mandeep Singh Baines <msb@...gle.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, fweisbec@...il.com
Subject: Re: PATCH: softlockup: Fix hung_task_check_count sysctl

To disable hung_task at runtime:

# echo 0 > /proc/sys/kernel/hung_task_check_count

33 /*
34  * Zero means infinite timeout - no checking done:
35  */
36 unsigned long __read_mostly sysctl_hung_task_timeout_secs = 120;

For hung_task_check_count, 0 means there is no maximum so all tasks
are checked.

Anton Blanchard (anton@...ba.org) wrote:
> 
> I'm seeing spikes of up to 0.5ms in khungtaskd on a large machine. To reduce
> this source of jitter I tried setting hung_task_check_count to 0:
> 

Hmm, maybe khungtaskd should be run using SCHED_IDLE.

> # echo 0 > /proc/sys/kernel/hung_task_check_count
> 
> which didn't have the intended response. Change to a post increment of
> max_count, so a value of 0 means check 0 tasks.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>
> ---
> 
> Index: linux.trees.git/kernel/hung_task.c
> ===================================================================
> --- linux.trees.git.orig/kernel/hung_task.c	2009-11-27 13:11:46.000000000 +1100
> +++ linux.trees.git/kernel/hung_task.c	2009-11-27 13:11:57.000000000 +1100
> @@ -144,7 +144,7 @@ static void check_hung_uninterruptible_t
>  
>  	rcu_read_lock();
>  	do_each_thread(g, t) {
> -		if (!--max_count)
> +		if (!max_count--)
>  			goto unlock;
>  		if (!--batch_count) {
>  			batch_count = HUNG_TASK_BATCHING;
--
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