linux-kernel - Re: [PATCH v2 3/3] mm/mempolicy: introduce MPOL_WEIGHTED

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Message-ID: <ZbAvR+U+tyLvsh8R@memverge.com>
Date: Tue, 23 Jan 2024 16:27:35 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	corbet@....net, akpm@...ux-foundation.org, honggyu.kim@...com,
	rakie.kim@...com, hyeongtak.ji@...com, mhocko@...nel.org,
	vtavarespetr@...ron.com, jgroves@...ron.com,
	ravis.opensrc@...ron.com, sthanneeru@...ron.com,
	emirakhur@...ron.com, Hasan.Maruf@....com, seungjun.ha@...sung.com,
	hannes@...xchg.org, dan.j.williams@...el.com,
	Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v2 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
 for weighted interleaving

On Tue, Jan 23, 2024 at 04:35:19PM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@...verge.com> writes:
> 
> > On Mon, Jan 22, 2024 at 11:54:34PM -0500, Gregory Price wrote:
> >> > 
> >> > Can the above code be simplified as something like below?
> >> > 
> >> >         resume_node = prev_node;
> > ---         resume_weight = 0;
> > +++         resume_weight = weights[node];
> >> >         for (...) {
> >> >                 ...
> >> >         }
> >> > 
> >> 
> >> I'll take another look at it, but this logic is annoying because of the
> >> corner case:  me->il_prev can be NUMA_NO_NODE or an actual numa node.
> >> 
> >
> > After a quick look, as long as no one objects to (me->il_prev) remaining
> > NUMA_NO_NODE
> 
> MAX_NUMNODES-1 ?
> 

When setting a new policy, the il_prev gets set to NUMA_NO_NODE. It's
not harmful and is just (-1), which is functionally the same as
(MAX_NUMNODES-1) for the purpose of iterating the nodemask with
next_node_in(). So it's fine to set (resume_node = me->il_prev)
as discussed.

I have a cleaned up function I'll push when i fix up a few other spots.

> > while having a weight assigned to pol->wil.cur_weight,
> 
> I think that it is OK.
> 
> And, IIUC, pol->wil.cur_weight can be 0, as in
> weighted_interleave_nodes(), if it's 0, it will be assigned to default
> weight for the node.
> 

cur_weight is different than the global weights.  cur_weight tells us
how many pages are remaining to allocate for the current node.

(cur_weight = 0) can happen in two scenarios:
  - initial setting of mempolicy (NUMA_NO_NODE w/ cur_weight=0)
  - weighted_interleave_nodes decrements it down to 0

Now that i'm looking at it - the second condition should not exist, and
we can eliminate it. The logic in weighted_interleave_nodes is actually
annoyingly unclear at the moment, so I'm going to re-factor it a bit to
be more explicit.

~Gregory