linux-kernel - Re: [PATCH 0/4] sched: Various reweight

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Message-ID: <20260210181308.GO2995752@noisy.programming.kicks-ass.net>
Date: Tue, 10 Feb 2026 19:13:08 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Doug Smythies <dsmythies@...us.net>
Cc: 'K Prateek Nayak' <kprateek.nayak@....com>, mingo@...nel.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
	wangtao554@...wei.com, quzicheng@...wei.com,
	wuyun.abel@...edance.com
Subject: Re: [PATCH 0/4] sched: Various reweight_entity() fixes

On Tue, Feb 03, 2026 at 08:36:41AM -0800, Doug Smythies wrote:

> Further to my email from the other day, where all was good [1],
> I have continued to test, in particular the severe overload conditions
> from [2].

> Conditions:
> Greater than 12,500 X (yes > /dev/null) tasks
> But less than 15,000 X ( yes > /dev/null) tasks
> 
> I have tested up to 20,000 X (yes > /dev/null) tasks
> with previous kernels, including mainline 6.19-rc1.
> 
> I would not disagree if you say my operating conditions
> are ridiculous.

They absolutely are; however!, people do crazy things so I doubt you are
alone.

> System:
> Processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz, 6 cores 12 CPUs.
> CPU frequency scaling driver: intel_pstate; Governor powersave.

Right, so I was too lazy to find a matching test machine, but instead
used taskset to limit myself to 6 cores/12 threads and let it rip.

# taskset -c -p 0-5,24-29 $$
# for ((i=0; i<20000; i++)) do yes > /dev/null & done

... a *LONG* while later ...

And I have reached 15k.

... this is *SLOW* ...

So I reached 20000 and figured what the heck and went for another 5k.

Eventually I managed to reach 21160, and then boom.

It is one of those pick_next_task_fair() NULL pointer derefs that are so
very indicative of math overflow.

I'll try and have a poke, if only this were a faster thing ;-)