Re: [PATCH 1/2] cgroup/cpuset: record DL BW alloc CPU for attach rollback

[Juri Lelli]

Hi!

On 2026-04-19 22:31:29-04:00, Waiman Long wrote:
> On 4/19/26 10:21 PM, Guopeng Zhang wrote:
> 
> > 在 2026/4/18 2:51, Waiman Long 写道:
> > ...
> > Hi Waiman,
> >
> > Thank you for the review and for the Reviewed-by.
> > I think you are right to call this out. Looking at the
> > current logic, !cpumask_intersects(oldcs->effective_cpus, cs->effective_cpus)
> > does not obviously guarantee that the migration is crossing into a different
> > root domain. If the old and new cpusets are disjoint but still belong to the
> > same root domain, it does look possible that we reserve bandwidth on the
> > destination side without a corresponding subtraction from the source side.
> > I will try to reproduce that configuration and follow up with results.
> > my current understanding is that the DL bandwidth
> > accounting is done at root-domain granularity, not at arbitrary cpuset-subset
> > granularity.
> 
> That is my understanding too.
> 
> > That also seems consistent with
> > Documentation/scheduler/sched-deadline.rst, which says that deadline tasks
> > cannot have a CPU affinity mask smaller than the root domain they are created
> > on, and that a restricted CPU set should be achieved by creating a restricted
> > root domain with cpuset.
> 
> A root domain should be created by creating cpuset root partition for v2 
> or using the cpuset.cpu_exclusive flag in v1.
> 
> What is listed in the documentation is the ideal case, but users may not 
> strictly follow the rule.

But, if they don't and try to create DEADLINE task on cpusets that are
subsets of a root-domain, that should fail, as the affinity mask won't
be covering the entire root domain. So no BW allocated (and no
additional data structures) for subsets either.

Thanks,
Juri