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

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Date:	Mon, 30 Nov 2009 12:21:20 -0800
From:	Mandeep 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

On Mon, Nov 30, 2009 at 11:13 AM, Mandeep Singh Baines <msb@...gle.com> wrote:
> To disable hung_task at runtime:
>
> # echo 0 > /proc/sys/kernel/hung_task_check_count
>

D'oh. Copy and paste error. I meant

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

The nice thing about hung_task_timeout_secs=0 is that hungtaskd will
no longer run. Implementing
no checking done with hung_task_check_count=0 would result in the task
getting scheduled,
running, and then doing nothing.

> 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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