linux-kernel - Re: [PATCH] hung_task: Skip scan on idle systems

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Message-ID: <3pafybrou47nar5l2xxqjvmbhupmsxr4vvawhdadd5qeyhu3sf@mmugthbxe2cs>
Date: Mon, 26 Jan 2026 15:14:27 -0500
From: Aaron Tomlin <atomlin@...mlin.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: neelx@...e.com, sean@...e.io, pmladek@...e.com, mhiramat@...nel.org, 
	akpm@...ux-foundation.org, joel.granados@...nel.org, mproche@...il.com, chjohnst@...il.com, 
	nick.lange@...il.com, gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hung_task: Skip scan on idle systems

On Mon, Jan 26, 2026 at 01:23:01PM +0800, Lance Yang wrote:
> Hi Aaron,

Hi Lance,

> Keep one patch or series under review at a time, especially in the
> same subsystem ...

Understood. That's fair.

> > @@ -503,6 +504,7 @@ static int watchdog(void *dummy)
> >   	for ( ; ; ) {
> >   		unsigned long timeout = sysctl_hung_task_timeout_secs;
> >   		unsigned long interval = sysctl_hung_task_check_interval_secs;
> > +		unsigned long load[3];
> >   		long t;
> >   		if (interval == 0)
> > @@ -511,8 +513,12 @@ static int watchdog(void *dummy)
> >   		t = hung_timeout_jiffies(hung_last_checked, interval);
> >   		if (t <= 0) {
> >   			if (!atomic_xchg(&reset_hung_task, 0) &&
> > -			    !hung_detector_suspended)
> > -				check_hung_uninterruptible_tasks(timeout);
> > +			    !hung_detector_suspended) {
> > +				/* Check 1-min load to detect idle system */
> > +				get_avenrun(load, 0, 0);
> > +				if (load[0] > 0)
> > +					check_hung_uninterruptible_tasks(timeout);
> 
> The optimization is not worth the trouble.
> 
> I don't think the assumption that "load[0] == 0 means no hung tasks" is
> 100% correct.
> 
> So that would miss actual hung tasks - a false negative, which is worse
> than the "wasted scan" you're trying to avoid.
> 
> Also, I don't *really* care about optimizing something that runs once
> every 120 seconds :)
> 
> Nacked-by: Lance Yang <lance.yang@...ux.dev>

Yes, please ignore. This is indeed wrong.

Regarding the value of the optimisation, while a 120-second interval
implies a low frequency, the cost of the scan is O(N). On large servers
with high thread counts (even if idle), iterating the entire task list
dirties cache lines and consumes memory bandwidth unnecessarily.

Nevertheless, we currently do not have a way to economically compute the
total number of tasks in TASK_UNINTERRUPTIBLE state.


Kind regards,
-- 
Aaron Tomlin

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