linux-kernel - Re: [RFC PATCH 12/20] kthread: Implement preferred affinity

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Message-ID: <8f5d3a92-f90c-4a0d-bd24-10dc49235afe@suse.cz>
Date: Tue, 6 Aug 2024 13:08:39 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 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: [RFC PATCH 12/20] kthread: Implement preferred affinity

On 8/6/24 01:59, Frederic Weisbecker wrote:
> On Mon, Aug 05, 2024 at 11:25:59PM +0200, Vlastimil Babka wrote:
>> > It's too bad we don't have a way to have a cpumask_possible_of_node(). I've
>> > looked into the guts of numa but that doesn't look easy to do.
>> 
>> That was my impression as well. Maybe not even possible because exact cpu
>> ids might not be pre-determined like this?
> 
> Probably.
> 
>> 
>> > Or there could be kthread_set_preferred_node()... ?
>> 
>> Possible instead of the callback idea suggested above?
>> kthreads_hotplug_update() could check if this is set and construct the mask
>> accordingly.
> 
> Or even better, callers of kthread_create_on_node() with actual node passed
> (!NUMA_NO_NODE) can be preferrably affined to the corresponding node by default
> unless told otherwise (that is unless kthread_bind() or
> kthread_set_preferred_affinity() has been called before the first wake up, and
> that includes kthread_create_on_cpu()).

Sounds logical and great!

> There are a few callers concerned: kswapd, kcompactd, some drivers:
> drivers/block/mtip32xx/mtip32xx.c, drivers/firmware/stratix10-svc.c,
> kernel/dma/map_benchmark.c, net/sunrpc/svc.c
> 
> After all kthread_create_on_cpu() affines to the corresponding CPU. So
> it sounds natural that kthread_create_on_node() affines to the corresponding
> node.

Yes.

> And then it's handled on hotplug just as a special case of preferred affinity.
> 
> Or is there something that wouldn't make that work?

Hopefully not.

> Thanks.
> 
> 
>> 
>> > Thanks.
>> > 
>> >> 
>> >> > Thanks.
>> >> 
>>