Re: [PATCH 0/7] sched/fair|isolation: Correctly clear nohz.[nr_cpus|idle_cpus_mask] for isolated CPUs

[Pierre Gondois]

Hello Waiman,
Thanks for the link, I didn't see the patchset previously.

On 4/4/24 05:01, Waiman Long wrote:
> On 4/3/24 11:05, Pierre Gondois wrote:
> > Zhang Rui reported that find_new_ilb() was iterating over CPUs in
> > isolated cgroup partitions. This triggered spurious wakeups for
> > theses CPUs. [1]
> > The initial approach was to ignore CPUs on NULL sched domains, as
> > isolated CPUs have a NULL sched domain. However a CPU:
> > - with its tick disabled, so taken into account in
> >     nohz.[idle_cpus_mask|nr_cpus]
> > - which is placed in an isolated cgroup partition
> > will never update nohz.[idle_cpus_mask|nr_cpus] again.
> >
> > To avoid that, the following variables should be cleared
> > when a CPU is placed in an isolated cgroup partition:
> > - nohz.idle_cpus_mask
> > - nohz.nr_cpus
> > - rq->nohz_tick_stopped
> > This would allow to avoid considering wrong nohz.* values during
> > idle load balance.
> >
> > As suggested in [2] and to avoid calling nohz_balance_[enter|exit]_idle()
> > from a remote CPU and create concurrency issues, leverage the existing
> > housekeeping HK_TYPE_SCHED mask to reflect isolated CPUs (i.e. on NULL
> > sched domains).
> > Indeed the HK_TYPE_SCHED mask is currently never set by the
> > isolcpus/nohz_full kernel parameters, so it defaults to cpu_online_mask.
> > Plus it's current usage fits CPUs that are isolated and should
> > not take part in load balancing.
> >
> > Making use of HK_TYPE_SCHED for this purpose implies creating a
> > housekeeping mask which can be modified at runtime.
> >
> > [1] https://lore.kernel.org/all/20230804090858.7605-1-rui.zhang@xxxxxxxxx/
> > [2] https://lore.kernel.org/all/CAKfTPtAMd_KNKhXXGk5MEibzzQUX3BFkWgxtEW2o8FFTX99DKw@xxxxxxxxxxxxxx/
> >
> > Pierre Gondois (7):
> >     sched/isolation: Introduce housekeeping_runtime isolation
> >     sched/isolation: Move HK_TYPE_SCHED to housekeeping runtime
> >     sched/isolation: Use HKR_TYPE_SCHED in find_new_ilb()
> >     sched/fair: Move/add on_null_domain()/housekeeping_cpu() checks
> >     sched/topology: Remove CPUs with NULL sd from HKR_TYPE_SCHED mask
> >     sched/fair: Remove on_null_domain() and redundant checks
> >     sched/fair: Clear idle_cpus_mask for CPUs with NULL sd
> >
> >    include/linux/sched/isolation.h | 30 ++++++++++++++++++++-
> >    include/linux/sched/nohz.h      |  2 ++
> >    kernel/sched/fair.c             | 44 +++++++++++++++++-------------
> >    kernel/sched/isolation.c        | 48 ++++++++++++++++++++++++++++++++-
> >    kernel/sched/topology.c         |  7 +++++
> >    5 files changed, 110 insertions(+), 21 deletions(-)
> I had also posted a patch series on excluding isolated CPUs in isolated
> partitions from housekeeping cpumasks earlier this year. See
>
> https://lore.kernel.org/lkml/20240229021414.508972-1-longman@xxxxxxxxxx/
>
> It took a different approach from this series. It looks like I should
> include HK_TYPE_MISC as well.

The common point between the 2 patchset is that find_new_ilb() won't
take into account isolated CPUs.
The present patchset also:
- clears nohz.[idle_cpus_mask|nr_cpus] variable when a CPU becomes isolated,
  cf. [PATCH 7/7] sched/fair: Clear idle_cpus_mask for CPUs with NULL sd
- tries to clean up/gather on_null_domain()/HK_TYPE_SCHED/HK_TYPE_MISC
  mask checks, as HK_TYPE_SCHED/HK_TYPE_MISC masks are currently never
  set.
but it also:
- updates the housekeeping mask from sched/topology.c. It might be better
  to do it from cpuset.c as you did as the update originally comes from
  here and it is unlikely another place would require updating housekeeping
  CPUs.
  A new housekeeping_runtime type is also created, but I think the way you
  handle updating housekeeping mask at runtime is better.
- adds a dependency of sched/fair.c over CPU_ISOLATION (cf. housekeeping_*
  calls), as Peter noted (IIUC) [1].

Should I re-spin the patchset and try to correct those points ? Or do you
think this should be done differently ?

Regards,
Pierre

[1] https://lore.kernel.org/lkml/20240404072745.GA35684@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

> Cheers,
> Longman