Re: [PATCH v2] sched: Store restrict_cpus_allowed_ptr() call state

Waiman Long
2023-01-26T20:50:07Z

On 1/26/23 11:11, Will Deacon wrote:
> On Tue, Jan 24, 2023 at 03:24:36PM -0500, Waiman Long wrote:
>> On 1/24/23 14:48, Will Deacon wrote:
>>> On Fri, Jan 20, 2023 at 09:17:49PM -0500, Waiman Long wrote:
>>>> The user_cpus_ptr field was originally added by commit b90ca8badbd1
>>>> ("sched: Introduce task_struct::user_cpus_ptr to track requested
>>>> affinity"). It was used only by arm64 arch due to possible asymmetric
>>>> CPU setup.
>>>>
>>>> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
>>>> cpumask"), task_struct::user_cpus_ptr is repurposed to store user
>>>> requested cpu affinity specified in the sched_setaffinity().
>>>>
>>>> This results in a performance regression in an arm64 system when booted
>>>> with "allow_mismatched_32bit_el0" on the command-line. The arch code will
>>>> (amongst other things) calls force_compatible_cpus_allowed_ptr() and
>>>> relax_compatible_cpus_allowed_ptr() when exec()'ing a 32-bit or a 64-bit
>>>> task respectively. Now a call to relax_compatible_cpus_allowed_ptr()
>>>> will always result in a __sched_setaffinity() call whether there is a
>>>> previous force_compatible_cpus_allowed_ptr() call or not.
>>> I'd argue it's more than just a performance regression -- the affinity
>>> masks are set incorrectly, which is a user visible thing
>>> (i.e. sched_getaffinity() gives unexpected values).
>> Can your elaborate a bit more on what you mean by getting unexpected
>> sched_getaffinity() results? You mean the result is wrong after a
>> relax_compatible_cpus_allowed_ptr(). Right?
> Yes, as in the original report. If, on a 4-CPU system, I do the following
> with v6.1 and "allow_mismatched_32bit_el0" on the kernel cmdline:
>
> # for c in `seq 1 3`; do echo 0 > /sys/devices/system/cpu/cpu$c/online; done
> # yes > /dev/null &
> [1] 334
> # taskset -p 334
> pid 334's current affinity mask: 1
> # for c in `seq 1 3`; do echo 1 > /sys/devices/system/cpu/cpu$c/online; done
> # taskset -p 334
> pid 334's current affinity mask: f
>
> but with v6.2-rc5 that last taskset invocation gives:
>
> pid 334's current affinity mask: 1
>
> so, yes, the performance definitely regresses, but that's because the
> affinity mask is wrong!

I see what you mean now. Hotplug doesn't work quite well now because
user_cpus_ptr has been repurposed to store the value set of
sched_setaffinity() but not the previous cpus_mask before
force_compatible_cpus_allowed_ptr().

One possible solution is to modify the hotplug related code to check for
the cpus_allowed_restricted, and if set, check task_cpu_possible_mask()
to see if the cpu can be added back to its cpus_mask. I will take a
further look at that later.

Cheers,
Longman