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

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Date:   Tue, 31 Jan 2023 21:22:44 -0500
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Cc:     kernel-team@...roid.com, Peter Zijlstra <peterz@...radead.org>,
        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 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.

Another suggestion that I have is to add a helper like 
has_task_cpu_possible_mask() that returns a true/false value. In this 
way, we only need to do an additional masking if we have some mismatched 
32-bit only cpus available in the system running 32-bit binaries so that 
we can skip this step in most cases compiling them out in non-arm64 systems.

By doing that, we may be able to remove some of the existing usages of 
task_cpu_possible_mask().

Thought?

Cheers,
Longman