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

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

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.