Re: [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter offline CPUs

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 01, 2023 at 01:46:11PM -0500, Waiman Long wrote:

> Note that using cpus_allowed directly in cgroup v2 may not be right because
> cpus_allowed may have no relationship to effective_cpus at all in some
> cases, e.g.
> 
>    root
>     |
>     V
>     A (cpus_allowed = 1-4, effective_cpus = 1-4)
>     |
>     V
>     B (cpus_allowed = 5-8, effective_cpus = 1-4)
> 
> In the case of cpuset B, passing back cpus 5-8 as the allowed_cpus is wrong.

I think my patch as written does the right thing here. Since the
intersection of (1-4) and (5-8) is empty it will move up the hierarchy
and we'll end up with (1-4) from the cgroup side of things.

So the purpose of __cs_cpus_allowed() is to override the cpus_allowed of
the root set and force it to cpu_possible_mask.

Then cs_cpus_allowed() computes the intersection of cs->cpus_allowed and
all it's parents. This will, in the case of B above, result in the empty
mask.

Then cpuset_cpus_allowed() has a loop that starts with
task_cpu_possible_mask(), intersects that with cs_cpus_allowed() and if
the intersection of that and cpu_online_mask is empty, moves up the
hierarchy. Given cs_cpus_allowed(B) is the empty mask, we'll move to A.

Note that since we force the mask of root to cpu_possible_mask,
cs_cpus_allowed(root) will be a no-op and if we guarantee (in arch code)
that cpu_online_mask always has a non-empty intersection with
task_cpu_possible_mask(), this loop is guaranteed to terminate with a
viable mask.