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

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Date: Mon, 19 Feb 2024 11:10:26 +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 Sun, Feb 18, 2024 at 08:46:16AM +0100, Oscar Salvador wrote:
> 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"
> 
> Thinking some more, promote might be misleading, just something like
> "do not try to migrate memory to memoryless nodes".

Thank you. I will.

> As this is not a bug fix but an optimization, as we will fail anyways
> in migrate_misplaced_folio() when migrate_balanced_pgdat() notices that
> we do not have any memory on that code.
> 
> With the other comments addressed:
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de>

Thank you for reviewing.

	Byungchul

> > 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?
> > 
> > > 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."
> > 
> > 
> > > 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.
> > 
> > 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
> > 
> 
> -- 
> Oscar Salvador
> SUSE Labs