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

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Date:   Fri, 8 Jun 2018 14:58:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     penguin-kernel@...ove.SAKURA.ne.jp, 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 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.