linux-kernel - Re: [PATCH 12/19] kthread: Default affine kthread to its preferred NUMA node

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Message-ID: <4da2ef8b-3286-4304-8496-59a97e868045@suse.cz>
Date: Wed, 7 Aug 2024 19:01:23 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Frederic Weisbecker <frederic@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Joel Fernandes <joel@...lfernandes.org>, Boqun Feng <boqun.feng@...il.com>,
 Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org
Subject: Re: [PATCH 12/19] kthread: Default affine kthread to its preferred
 NUMA node

On 8/7/24 18:02, Frederic Weisbecker wrote:
> Kthreads attached to a preferred NUMA node for their task structure
> allocation can also be assumed to run preferrably within that same node.
> 
> A more precise affinity is usually notified by calling
> kthread_create_on_cpu() or kthread_bind[_mask]() before the first wakeup.
> 
> For the others, a default affinity to the node is desired and sometimes
> implemented with more or less success when it comes to deal with hotplug
> events and nohz_full / CPU Isolation interactions:
> 
> - kcompactd is affine to its node and handles hotplug but not CPU Isolation
> - kswapd is affine to its node and ignores hotplug and CPU Isolation
> - A bunch of drivers create their kthreads on a specific node and
>   don't take care about affining further.
> 
> Handle that default node affinity preference at the generic level
> instead, provided a kthread is created on an actual node and doesn't
> apply any specific affinity such as a given CPU or a custom cpumask to
> bind to before its first wake-up.
> 
> This generic handling is aware of CPU hotplug events and CPU isolation
> such that:
> 
> * When a housekeeping CPU goes up and is part of the node of a given
>   kthread, it is added to its applied affinity set (and
>   possibly the default last resort online housekeeping set is removed
>   from the set).
> 
> * When a housekeeping CPU goes down while it was part of the node of a
>   kthread, it is removed from the kthread's applied
>   affinity. The last resort is to affine the kthread to all online
>   housekeeping CPUs.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Acked-by: Vlastimil Babka <vbabka@...e.cz>