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

[Linux kernel regression tracking (#update)]

[TLDR: This mail in primarily relevant for Linux kernel regression
tracking. There afaics is a fix for the regression discussed in this
thread, but it did not use a Link: tag to point to the report, as Linus
and the documentation call for. Things happen, no worries -- but it
forces me to write this mail to make the regression tracking bot aware
of the fix. See link in footer if these mails annoy you.]

On 26.01.23 13:52, Linux kernel regression tracking (#adding) wrote:
> 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 fix: 3fb906e7fabbb5b76c3c
#regzbot ignore-activity

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