Date:   Wed, 1 Feb 2023 10:15:30 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org,
        kernel-team@android.com, Zefan Li <lizefan.x@bytedance.com>,
        Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>, cgroups@vger.kernel.org
Subject: Re: [PATCH 2/2] cpuset: Call set_cpus_allowed_ptr() with appropriate
 mask for task
Message-ID: <Y9otsg1ihF3LH2vG@hirez.programming.kicks-ass.net>
References: <20230131221719.3176-1-will@kernel.org>
 <20230131221719.3176-3-will@kernel.org>
 <66cdf2e8-f1aa-5dfe-cd2e-0e37dc0ae799@redhat.com>
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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@kernel.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*.