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Date:   Wed, 23 Nov 2022 16:19:44 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Waiman Long <longman@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] cgroup/cpuset: Optimize cpuset_attach() on v2
Message-ID: <Y346ENzM4+FAjRnn@linutronix.de>
References: <20221112221939.1272764-1-longman@redhat.com>
 <20221112221939.1272764-3-longman@redhat.com>
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On 2022-11-12 17:19:39 [-0500], Waiman Long wrote:
> It was found that with the default hierarchy, enabling cpuset in the
> child cgroups can trigger a cpuset_attach() call in each of the child
> cgroups that have tasks with no change in effective cpus and mems. If
> there are many processes in those child cgroups, it will burn quite a
> lot of cpu cycles iterating all the tasks without doing useful work.

Thank you.

So this preserves the CPU mask upon attaching the cpuset container.

| ~# taskset -pc $$
| pid 1564's current affinity list: 0-2

default mask after boot due to isolcpus=

| ~# echo "+cpu" >> /sys/fs/cgroup/cgroup.subtree_control ; echo "+cpuset" >> /sys/fs/cgroup/cgroup.subtree_control
| ~# taskset -pc $$
| pid 1564's current affinity list: 0-2

okay.

| ~# echo 1-3 > /sys/fs/cgroup/user.slice/cpuset.cpus
| ~# taskset -pc $$
| pid 1564's current affinity list: 1-3

wiped away.

| ~# taskset -pc 2-3 $$ 
| pid 1564's current affinity list: 1-3
| pid 1564's new affinity list: 2,3
| ~# echo 2-4 > /sys/fs/cgroup/user.slice/cpuset.cpus
| ~# taskset -pc 2-3 $$ 
| pid 1564's current affinity list: 2,3
| pid 1564's new affinity list: 2,3

But it works if the mask was changed on purpose.

Sebastian