Message-ID: <af7488b7-9bed-69c2-274f-90ce05f8b435@redhat.com>
Date:   Thu, 26 Jan 2023 15:49:12 -0500
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Subject: Re: [PATCH v2] sched: Store restrict_cpus_allowed_ptr() call state
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To:     Will Deacon <will@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Phil Auld <pauld@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, regressions@lists.linux.dev,
        regressions@leemhuis.info
References: <20230121021749.55313-1-longman@redhat.com>
 <20230124194805.GA27257@willie-the-truck>
 <fdf76fb2-1da4-2d72-7eb3-21137a7d6845@redhat.com>
 <20230126161110.GB29438@willie-the-truck>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20230126161110.GB29438@willie-the-truck>
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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