RE: [PATCH 0/4] sched: Various reweight_entity() fixes

["Doug Smythies" <dsmythies@telus.net>]

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On 2026.02.10 10:13 Peter Zijlstra wrote:
> 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* ...

Thanks for trying it.
And yes it gets very slow. I should have warned readers.
With the first version of this patch set it took my computer 20 minutes
to spin out 18,000 tasks. A graph is attached.
Note that more typically I could not get to 18,000 tasks. I don't know a
predictable way to create the hang. 

By the way, I had no issue with 80,000 tasks, if they contained some
regular sleep time. A graph is attached. The load average was 79,400.
 
> 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 ;-)

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