linux-kernel - Re: [PATCH v3] sched/numa, mm: do not promote folios to nodes not set N

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Message-ID: <20240219020854.GA65758@system.software.com>
Date: Mon, 19 Feb 2024 11:08:54 +0900
From: Byungchul Park <byungchul@...com>
To: Oscar Salvador <osalvador@...e.de>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	bristot@...hat.com, vschneid@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v3] sched/numa, mm: do not promote folios to nodes not
 set N_MEMORY

On Fri, Feb 16, 2024 at 02:51:24PM +0100, Oscar Salvador wrote:
> On Fri, Feb 16, 2024 at 08:40:45PM +0900, Byungchul Park wrote:
> > From 150af2f78e19217a1d03e47e3ee5279684590fb4 Mon Sep 17 00:00:00 2001
> > From: Byungchul Park <byungchul@...com>
> > Date: Fri, 16 Feb 2024 20:18:10 +0900
> > Subject: [PATCH v3] sched/numa, mm: do not promote folios to nodes not set N_MEMORY
> 
> "do not try to promote folios to memoryless nodes"

Thank you. I will.

> because AFAICS we are just trying.
> Even if should_numa_migrate_memory() returns true, I assume that we will
> fail somewhere down the chain e.g: migrate_pages() when we see that this
> node does not any memory, right?

Yes.

> > A numa node might not have its local memory but CPUs. Promoting a folio
> > to the node's local memory is nonsense. So avoid nodes not set N_MEMORY
> > from getting promoted.
> 
> If you talk about memoryless nodes everybody gets it better IMHO.
> "Memoryless nodes do not have any memory to migrate to, so stop trying it."

Much better.

> > Signed-off-by: Byungchul Park <byungchul@...com>
> > ---
> >  kernel/sched/fair.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d7a3c63a2171..7ed9ef3c0134 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1828,6 +1828,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
> >  	int dst_nid = cpu_to_node(dst_cpu);
> >  	int last_cpupid, this_cpupid;
> >  
> > +	/*
> > +	 * A node of dst_nid might not have its local memory. Promoting
> > +	 * a folio to the node is meaningless.
> > +	 */
> > +	if (!node_state(dst_nid, N_MEMORY))
> > +		return false;
> 
> "Cannot migrate to memoryless nodes"
> 
> seems shorter and more clear.

Agree.

	Byungchul

> So, what happens when we return true here? will we fail at
> migrate_pages() I guess? That is quite down the road so I guess
> this check can save us some time.
> 
> 
> -- 
> Oscar Salvador
> SUSE Labs