linux-kernel - Re: [PATCH] kernel/hung_task.c: allow to set period separately from timeout

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Message-ID: <4d9f8cd8-1b2c-28ef-1705-2400e5af9b51@i-love.sakura.ne.jp>
Date:   Sat, 9 Jun 2018 16:00:57 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        syzkaller@...glegroups.com
Subject: Re: [PATCH] kernel/hung_task.c: allow to set period separately from
 timeout

On 2018/06/09 6:58, Andrew Morton wrote:
> On Fri,  8 Jun 2018 15:30:43 +0200 Dmitry Vyukov <dvyukov@...gle.com> wrote:
> 
>> Currently task hung checking period is equal to timeout,
>> as the result hung is detected anywhere between timeout and 2*timeout.
>> This is fine for most interactive environments, but this hurts automated
>> testing setups (syzbot). In an automated setup we need to strictly order
>> CPU lockup < RCU stall < workqueue lockup < task hung < silent loss,
>> so that RCU stall is not detected as task hung and task hung is not
>> detected as silent machine loss. The large variance in task hung
>> detection timeout requires setting silent machine loss timeout to
>> a very large value (e.g. if task hung is 3 mins, then silent loss
>> need to be set to ~7 mins). The additional 3 minutes significantly
>> reduce testing efficiency because usually we crash kernel within
>> a minute, and this can add hours to bug localization process as it
>> needs to do dozens of tests.
>>
>> Allow setting checking period separately from timeout.
>> This allows to set timeout to, say, 3 minutes, but period to 10 secs.
>>
>> The period is controlled via a new hung_task_period_secs sysctl,
>> similar to the existing hung_task_timeout_secs sysctl.
>> The default value of 0 results in the current behavior.
> 
> I'm rather struggling to understand the difference between "period" and
> "timeout".  We would benefit from a clear description of what these two
> things do.  An appropriate place for this description is
> Documentation/sysctl/kernel.txt, which this patch forgot to update.

My understanding is that "period" is "how frequently we should check"
and "timeout" is "how long a thread remained uninterruptible". Maybe
hung_task_check_interval_secs would be better than hung_task_period_secs.

timeout = 60 and period = 1 would allow hung task to be reported as soon
as it remained uninterruptible for 60 seconds. That makes me easier to
narrow down relevant kernel messages and syzbot program.

Well, showing exact slept time, along with all threads which slept more
than some threshold (e.g. timeout / 2), might be helpful.