Re: [PATCH 1/1] cgroup/cpuset: Rebuild sched domains if isolated partition changed

[Waiman Long]

On 9/18/23 03:07, Pierre Gondois wrote:
> Hello Longman,
>
> On 9/17/23 18:51, Waiman Long wrote:
> > On 9/15/23 11:45, Pierre Gondois wrote:
> > > When an isolated parition is created, the sched domains (sds) are
> > > rebuilt and the sds of the isolated CPUs are detached. This only
> > > happens at the creation of the isolated parition. Updating
> > > the cpuset of the partition doesn't rebuild the sds:
> > >
> > > To reproduce:
> > >     # ls /sys/kernel/debug/sched/domains/cpu0/
> > >     domain0
> > >     # ls /sys/kernel/debug/sched/domains/cpu1/
> > >     domain0
> > >     #
> > >     # mkdir cgroup
> > >     # mount -t cgroup2 none cgroup/
> > >     # mkdir cgroup/A1/
> > >     # echo "+cpuset" > cgroup/cgroup.subtree_control
> > >     # echo 0-3 > cgroup/A1/cpuset.cpus
> > >     # echo isolated > cgroup/A1/cpuset.cpus.partition
> > >     #
> > >     # ls /sys/kernel/debug/sched/domains/cpu0/
> > >     # ls /sys/kernel/debug/sched/domains/cpu1/
> > >     #
> > >     # echo 0 > cgroup/A1/cpuset.cpus
> > >     # ls /sys/kernel/debug/sched/domains/cpu0/
> > >     # ls /sys/kernel/debug/sched/domains/cpu1/
> > >     #
> > >
> > > Here CPU1 should have a sched domain re-attached.
> > >
> > > Signed-off-by: Pierre Gondois <pierre.gondois@xxxxxxx>
> > > ---
> > >    kernel/cgroup/cpuset.c | 12 ++++++++----
> > >    1 file changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > index 58e6f18f01c1..e3eb27ff9b68 100644
> > > --- a/kernel/cgroup/cpuset.c
> > > +++ b/kernel/cgroup/cpuset.c
> > > @@ -1680,11 +1680,15 @@ static void update_cpumasks_hier(struct 
> > > cpuset *cs, struct tmpmasks *tmp,
> > >             * empty cpuset is changed, we need to rebuild sched 
> > > domains.
> > >             * On default hierarchy, the cpuset needs to be a partition
> > >             * root as well.
> > > +         * Also rebuild sched domains if the cpuset of an isolated
> > > +         * partition changed.
> > >             */
> > > -        if (!cpumask_empty(cp->cpus_allowed) &&
> > > -            is_sched_load_balance(cp) &&
> > > -           (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
> > > -            is_partition_valid(cp)))
> > > +        if ((!cpumask_empty(cp->cpus_allowed) &&
> > > +             is_sched_load_balance(cp) &&
> > > +             (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
> > > +              is_partition_valid(cp))) ||
> > > +            (cp->partition_root_state == PRS_ISOLATED ||
> > > +             cp->partition_root_state == PRS_INVALID_ISOLATED))
> > >                need_rebuild_sched_domains = true;
> > >               rcu_read_lock();
> >
> > Thanks for spotting the problem and sending out a patch to fix it.
> > However, it should be done in the update_cpumask(). The sched_domain
> > rebuild in update_cpumasks_hier() is supposed to be used for impacted
> > descendant cpusets lower down in the hierarchy.
> >
> > Anyway, I believe this problem should have been fixed in commit
> > a86ce68078b2 ("cgroup/cpuset: Extract out CS_CPU_EXCLUSIVE &
> > CS_SCHED_LOAD_BALANCE handling") recently merged into the v6.6 kernel.
> > Would you mind testing the latest upstream kernel to see if this problem
> > is gone or not?
>
> Yes right, v6.6-rc2 kernel doesn't have this issue. My bad for not 
> updating
> it ...
>
> However I think the second issue described in the cover letter can 
> still be
> reproduced on v6.6-rc2. This present patch was not aiming to fix it 
> though.
>
> Commands:
> # mkdir cgroup
> # mount -t cgroup2 none cgroup/
> # mkdir cgroup/A1 cgroup/B1
> # echo "+cpuset" > cgroup/cgroup.subtree_control
> # echo 0-3 > cgroup/A1/cpuset.cpus
> # echo isolated > cgroup/A1/cpuset.cpus.partition
> # echo 4-6 > cgroup/B1/cpuset.cpus
> # cat cgroup/A1/cpuset.cpus.partition
> isolated
> # echo 0-4 > cgroup/A1/cpuset.cpus
> # cat cgroup/A1/cpuset.cpus.partition
> isolated invalid (Cpu list in cpuset.cpus not exclusive)
> # echo 0-3 > cgroup/A1/cpuset.cpus
> # cat cgroup/A1/cpuset.cpus.partition
> isolated invalid (Cpu list in cpuset.cpus not exclusive)
>
> Cgroup A1 should be a valid isolated partition once its CPUs become
> exclusive again I believe,

Yes, the current code doesn't always detect that an invalid partition 
can be valid again and turn it back on. Will try to fix that. In the 
mean time, a workaround is to do

# echo member > cgroup/A1/cpuset.cpus.partition
# echo isolated > cgroup/A1/cpuset.cpus.partition

Thanks,
Longman