linux-kernel - Re: [PATCH v2] sched: Store restrict_cpus_allowed

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Date:   Fri, 27 Jan 2023 13:03:06 +0000
From:   Will Deacon <will@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
        regressions@...mhuis.info
Subject: Re: [PATCH v2] sched: Store restrict_cpus_allowed_ptr() call state

On Thu, Jan 26, 2023 at 08:56:51PM -0500, Waiman Long wrote:
> On 1/26/23 15:58, Waiman Long wrote:
> > On 1/26/23 15:49, Waiman Long wrote:
> > > 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.
> > 
> > Wait, I think the cpuset hotplug code should be able to restore the
> > right cpumask since task_cpu_possible_mask() is used there. Is cpuset
> > enabled? Does the test works without allow_mismatched_32bit_el0?
> 
> BTW, if the test result is from running on a kernel built with the v2 patch,
> it is the unexpected result. That should be fixed in the v3 patch.

The failure listed above is on vanilla v6.2-rc5. Your v2 has other issues,
as described in:

https://lore.kernel.org/r/20230124194805.GA27257@willie-the-truck

Will