linux-kernel - Re: [PATCH v8 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Message-ID: <20211116175411.GA50019@blackbody.suse.cz>
Date:   Tue, 16 Nov 2021 18:54:11 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v8 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

On Mon, Nov 15, 2021 at 04:10:29PM -0500, Waiman Long <longman@...hat.com> wrote:
> > On Mon, Oct 18, 2021 at 10:36:18AM -0400, Waiman Long <longman@...hat.com> wrote:
> > > +	scheduler.  Tasks in such a partition must be explicitly bound
> > > +	to each individual CPU.
> [...]
> 
> It can be a problem when one is trying to move from one cgroup to another
> cgroup with non-overlapping cpus laterally. However, if a task is initially
> from a parent cgroup with affinity mask that include cpus in the isolated
> child cgroup, I believe it should be able to move to the isolated child
> cgroup without problem. Otherwise, it is a bug that needs to be fixed.

app_root	cpuset.cpus=0-3
`- non_rt	cpuset.cpus=0-1	cpuset.cpus.partition=member
`- rt		cpuset.cpus=2-3	cpuset.cpus.partition=isolated

The app_root would have cpuset.cpus.effective=0-1 so even the task in
app_root can't sched_setaffinity() to cpus 2-3.
But AFAICS, the migration calls set_cpus_allowed_ptr() anyway, so the
task in the isolated partition needn't to bind explicitly with
sched_setaffinity(). (It'd have two cpus available, so one more
sched_setaffinity() or migration into a single-cpu list is desirable.)

All in all, I think the behavior is OK and the explicit binding of tasks
in an isolated cpuset is optional (not a must as worded currently).


> I think the wording may be confusing. What I meant is none of the requested
> cpu can be granted. So if there is at least one granted, the effective cpus
> won't be empty.

Ack.

> You currently cannot make change to cpuset.cpus that violates the cpu
> exclusivity rule. The above constraints will not disallow you to make the
> change. They just affect the validity of the partition root.

Sibling exclusivity should be a validity condition regardless of whether
transition is allowed or not. (At least it looks simpler to me.)


> > > +        Changing a partition root to "member" is always allowed.
> > > +        If there are child partition roots underneath it, however,
> > > +        they will be forced to be switched back to "member" too and
> > > +        lose their partitions. So care must be taken to double check
> > > +        for this condition before disabling a partition root.
> > (Or is this how delegation is intended?) However, AFAICS, parent still
> > can't remove cpuset.cpus even when the child is a "member". Otherwise,
> > I agree with the back-switch.
> There are only 2 possibilities here. Either we force the child partitions to
> be become members or invalid partition root.

My point here was mostly about preempting the cpus (as a v2 specific
feature). (I'm rather indifferent whether children turn into invalid
roots or members.)

Thanks,
Michal