linux-kernel - Re: [PATCH] hung_task: Allow printing warnings every check interval

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Message-ID: <41fd7652-df1f-26f6-aba0-b87ebae07db6@i-love.sakura.ne.jp>
Date:   Fri, 26 Jul 2019 22:28:53 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vasiliy Khoruzhick <vasilykh@...sta.com>,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] hung_task: Allow printing warnings every check interval

On 2019/07/26 20:29, Tetsuo Handa wrote:
> On 2019/07/25 23:25, Dmitry Safonov wrote:
>> Yes, also current distributions already using the counter to print
>> warnings number of times and then silently ignore. I.e., on my Arch
>> Linux setup:
>> hung_task_warnings:10
> 
> You can propose changing the default value of hung_task_warnings to -1.
> 
> Current patch might be inconvenient because printk() from hung_task_warning(t, false)
> fails to go to consoles when that "t" was blocked for more than "timeout" seconds, for
> 
> 	if (sysctl_hung_task_panic) {
> 		console_verbose();
> 		hung_task_show_lock = true;
> 		hung_task_call_panic = true;
> 	}
> 
> path which is intended to force printk() to go to consoles is ignored by
> 
> 	/* Don't print warings twice */
> 	if (!sysctl_hung_task_interval_warnings)
> 		hung_task_warning(t, true);
> 
> when panic() should be called. (The vmcore would contain the printk() output which
> was not sent to consoles if kdump is configured. But vmcore is not always available.)
> 
>> Yes, that's why it's disabled by default (=0).
>> I tend to agree that printing with KERN_DEBUG may be better, but in my
>> point of view the patch isn't enough justification for patching
>> sched_show_task() and show_stack().
> 
> You can propose sched_show_task_log_lvl() and show_stack_log_lvl() like show_trace_log_lvl().
> 
> I think that sysctl_hung_task_interval_warnings should not be decremented automatically.
> I guess that that variable should become a boolean which controls whether to report threads
> (with KERN_DEBUG level) which was blocked for more than sysctl_hung_task_check_interval_secs
> seconds (or a tristate which also controls whether the report should be sent to consoles
> (because KERN_DEBUG level likely prevents sending to consoles)), and
> hung_task_warning(t, false) should be called like
> 
> 	if (time_is_after_jiffies(t->last_switch_time + timeout * HZ)) {
> 		if (sysctl_hung_task_interval_warnings)
> 			hung_task_warning(t, false);
> 		return;
> 	}
> 
> rather than
> 
> 	if (sysctl_hung_task_interval_warnings)
> 		hung_task_warning(t, false);
> 	if (time_is_after_jiffies(t->last_switch_time + timeout * HZ))
> 		return;
> 
> .
> 


Well, another direction is to disassociate sysctl_hung_task_panic from
sysctl_hung_task_timeout_secs. Since nobody would want to call panic() when
a thread was blocked for only one second, allow sysctl_hung_task_panic to
specify larger than 1, and interpret it as sysctl_hung_task_timeout_secs for
calling panic(). Roughly speaking:

-	if (sysctl_hung_task_panic) {
+	unsigned long panic_timeout = READ_ONCE(sysctl_hung_task_panic)
+	if (panic_timeout == 1 || (panic_timeout > 1 &&
+	     (jiffies - t->last_switch_time) / HZ >= panic_timeout)) {
 		console_verbose();
 		hung_task_show_lock = true;
 		hung_task_call_panic = true;
 	}

If use of different loglevel is not a requirement for you, this would be the simplest.