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

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Message-ID: <ZrFncpgfJj9sWuDO@lothringen>
Date: Tue, 6 Aug 2024 01:59:46 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Vlastimil Babka <vbabka@...e.cz>
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 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()).

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.

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?

Thanks.


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