linux-kernel - Re: [PATCH 2/2] cpuset: Call set_cpus_allowed

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Date:   Wed, 1 Feb 2023 10:03:27 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Zefan Li <lizefan.x@...edance.com>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH 2/2] cpuset: Call set_cpus_allowed_ptr() with appropriate
 mask for task

On 2/1/23 04:15, Peter Zijlstra wrote:
> On Tue, Jan 31, 2023 at 09:22:44PM -0500, Waiman Long wrote:
>> On 1/31/23 17:17, Will Deacon wrote:
>>> set_cpus_allowed_ptr() will fail with -EINVAL if the requested
>>> affinity mask is not a subset of the task_cpu_possible_mask() for the
>>> task being updated. Consequently, on a heterogeneous system with cpusets
>>> spanning the different CPU types, updates to the cgroup hierarchy can
>>> silently fail to update task affinities when the effective affinity
>>> mask for the cpuset is expanded.
>>>
>>> For example, consider an arm64 system with 4 CPUs, where CPUs 2-3 are
>>> the only cores capable of executing 32-bit tasks. Attaching a 32-bit
>>> task to a cpuset containing CPUs 0-2 will correctly affine the task to
>>> CPU 2. Extending the cpuset to CPUs 0-3, however, will fail to extend
>>> the affinity mask of the 32-bit task because update_tasks_cpumask() will
>>> pass the full 0-3 mask to set_cpus_allowed_ptr().
>>>
>>> Extend update_tasks_cpumask() to take a temporary 'cpumask' paramater
>>> and use it to mask the 'effective_cpus' mask with the possible mask for
>>> each task being updated.
>>>
>>> Fixes: 431c69fac05b ("cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()")
>>> Signed-off-by: Will Deacon <will@...nel.org>
>>> ---
>>>
>>> Note: We wondered whether it was worth calling guarantee_online_cpus()
>>> if the cpumask_and() returns 0 in update_tasks_cpumask(), but given that
>>> this path is only called when the effective mask changes, it didn't
>>> seem appropriate. Ultimately, if you have 32-bit tasks attached to a
>>> cpuset containing only 64-bit cpus, then the affinity is going to be
>>> forced.
>> Now I see how the sched_setaffinity() change is impacting arm64. Instead of
>> putting in the bandage in cpuset. I would suggest doing another cpu masking
>> in __set_cpus_allowed_ptr() similar to what is now done for user_cpus_ptr.
> NO! cpuset is *BROKEN* it has been for a while, it needs to get fixed.
>
> Masking the offline CPUs is *WRONG*.
>
This patch is not related to offline cpus at all. It is all about the 
32-bit misfit cpus in some arm64 system.

Cheers,
Longman