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

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Message-ID: <9a919c32-4e0e-ca1b-887f-c329543913d3@i-love.sakura.ne.jp>
Date:   Fri, 26 Jul 2019 20:29:52 +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 2019/07/25 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;

.