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

Will Deacon
2023-01-26T16:11:47Z

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!

Will