linux-kernel - Re: [PATCH v2] kernel/hung_task.c: allow to set checking interval separately from timeout

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Date:   Tue, 19 Jun 2018 16:03:27 -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 v2] kernel/hung_task.c: allow to set checking interval
 separately from timeout

On Mon, 11 Jun 2018 13:10:04 +0200 Dmitry Vyukov <dvyukov@...gle.com> wrote:

> Currently task hung checking interval 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 interval separately from timeout.
> This allows to set timeout to, say, 3 minutes,
> but checking interval to 10 secs.
> 
> The interval is controlled via a new hung_task_check_interval_secs
> sysctl, similar to the existing hung_task_timeout_secs sysctl.
> The default value of 0 results in the current behavior:
> checking interval is equal to timeout.

I suppose we shoold do this:

--- a/kernel/sysctl.c~kernel-hung_taskc-allow-to-set-checking-interval-separately-from-timeout-fix
+++ a/kernel/sysctl.c
@@ -145,7 +145,10 @@ static int minolduid;
 static int ngroups_max = NGROUPS_MAX;
 static const int cap_last_cap = CAP_LAST_CAP;
 
-/*this is needed for proc_doulongvec_minmax of sysctl_hung_task_timeout_secs */
+/*
+ * This is needed for proc_doulongvec_minmax of sysctl_hung_task_timeout_secs
+ * and hung_task_check_interval_secs
+ */
 #ifdef CONFIG_DETECT_HUNG_TASK
 static unsigned long hung_task_timeout_max = (LONG_MAX/HZ);
 #endif