linux-kernel - Re: [PATCH 0/7] sched/fair|isolation: Correctly clear nohz.[nr_cpus|idle_cpus

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Date: Wed, 3 Apr 2024 23:01:03 -0400
From: Waiman Long <longman@...hat.com>
To: Pierre Gondois <pierre.gondois@....com>, linux-kernel@...r.kernel.org
Cc: Aaron Lu <aaron.lu@...el.com>, Rui Zhang <rui.zhang@...el.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 0/7] sched/fair|isolation: Correctly clear
 nohz.[nr_cpus|idle_cpus_mask] for isolated CPUs

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@intel.com/
> [2] https://lore.kernel.org/all/CAKfTPtAMd_KNKhXXGk5MEibzzQUX3BFkWgxtEW2o8FFTX99DKw@mail.gmail.com/
>
> 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@redhat.com/

It took a different approach from this series. It looks like I should 
include HK_TYPE_MISC as well.

Cheers,
Longman