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Date: Thu, 4 Apr 2024 20:23:27 -0400
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Subject: Re: [PATCH 0/7] sched/fair|isolation: Correctly clear
 nohz.[nr_cpus|idle_cpus_mask] for isolated CPUs
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To: Pierre Gondois <pierre.gondois@arm.com>, linux-kernel@vger.kernel.org
Cc: Aaron Lu <aaron.lu@intel.com>, Rui Zhang <rui.zhang@intel.com>,
 Anna-Maria Behnsen <anna-maria@linutronix.de>,
 Frederic Weisbecker <frederic@kernel.org>, Ingo Molnar <mingo@kernel.org>,
 Thomas Gleixner <tglx@linutronix.de>, Peter Zijlstra <peterz@infradead.org>,
 Juri Lelli <juri.lelli@redhat.com>,
 Vincent Guittot <vincent.guittot@linaro.org>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>,
 Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>,
 Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira
 <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>,
 Tejun Heo <tj@kernel.org>, Michal Hocko <mhocko@suse.com>
References: <20240403150543.2793354-1-pierre.gondois@arm.com>
 <98443f19-c653-493e-a2a9-e1d07b9d8468@redhat.com>
 <cecd7e83-b83c-416e-b59b-296f84e5c574@arm.com>
From: Waiman Long <longman@redhat.com>
In-Reply-To: <cecd7e83-b83c-416e-b59b-296f84e5c574@arm.com>
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On 4/4/24 08:55, Pierre Gondois wrote:
> 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@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.
>
> 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].

That is true. Without CONFIG_CPU_ISOLATION, all the housekeeping* calls 
are essentially no-ops.

OTOH, without CONFIG_CPU_ISOLATION, a number of isolation capabilities 
won't be there. Most distros will have this config option set anyway.

BTW, a number of the HK_TYPE_* are also used at runtime, like 
HK_TYPE_TIMER and HK_TYPE_RCU. So it is hard to artificially distinguish 
between runtime or boot time.

I don't believe you need to add direct dependency on 
CONFIG_CPU_ISOLATION, but you do have to add any housekeeping check as 
an additional check, not as a replacement of the existing check.

Cheers,
Longman