Re: [PATCH v10 2/5] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity()

[Linux kernel regression tracking (#adding)]

[TLDR: I'm adding this report to the list of tracked Linux kernel
regressions; the text you find below is based on a few templates
paragraphs you might have encountered already in similar form.
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On 17.01.23 17:08, Will Deacon wrote:
> 
> On Thu, Sep 22, 2022 at 02:00:38PM -0400, Waiman Long wrote:
>> The user_cpus_ptr field is added by commit b90ca8badbd1 ("sched:
>> Introduce task_struct::user_cpus_ptr to track requested affinity"). It
>> is currently used only by arm64 arch due to possible asymmetric CPU
>> setup. This patch extends its usage to save user provided cpumask
>> when sched_setaffinity() is called for all arches. With this patch
>> applied, user_cpus_ptr, once allocated after a successful call to
>> sched_setaffinity(), will only be freed when the task exits.
> [...]
> We've tracked this down as the cause of an arm64 regression in Android and I've
> reproduced the issue with mainline.
> 
> Basically, if an arm64 system is booted with "allow_mismatched_32bit_el0" on
> the command-line, then the arch code will (amongst other things) call
> force_compatible_cpus_allowed_ptr() and relax_compatible_cpus_allowed_ptr()
> when exec()'ing a 32-bit or a 64-bit task respectively.
> [...]

Thanks for the report. To be sure the issue doesn't fall through the
cracks unnoticed, I'm adding it to regzbot, the Linux kernel regression
tracking bot:

#regzbot ^introduced 8f9ea86fdf99
#regzbot title sched: incorrectly set affinity masks and performance
regression on arm64
#regzbot monitor:
https://lore.kernel.org/all/20230121021749.55313-1-longman@xxxxxxxxxx/
#regzbot ignore-activity

This isn't a regression? This issue or a fix for it are already
discussed somewhere else? It was fixed already? You want to clarify when
the regression started to happen? Or point out I got the title or
something else totally wrong? Then just reply and tell me -- ideally
while also telling regzbot about it, as explained by the page listed in
the footer of this mail.

Developers: When fixing the issue, remember to add 'Link:' tags pointing
to the report (the parent of this mail). See page linked in footer for
details.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
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