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

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Date: Mon, 22 Jan 2024 23:54:34 -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 11:02:03AM +0800, Huang, Ying wrote:
> Gregory Price <gourry.memverge@...il.com> writes:
> 
> > +	int prev_node = NUMA_NO_NODE;
> 
> It appears that we should initialize prev_node with me->il_prev?
> Details are as below.
> 

yeah good catch, was a rebase error from my tested code, where this is
the case.  patching now.

> > +		if (rem_pages <= pol->wil.cur_weight) {
> > +			pol->wil.cur_weight -= rem_pages;
> 
> If "pol->wil.cur_weight == 0" here, we need to change me->il_prev?
> 
you are right, and also need to fetch the next cur_weight.  Seems I
missed this specific case in my tests.  (had this tested with a single
node but not 2, so it looked right).

Added to my test suite.

> We can replace "weight_nodes" with "i" and use a "for" loop?
> 
> > +	while (weight_nodes < nnodes) {
> > +		node = next_node_in(prev_node, nodes);
> 
> IIUC, "node" will not change in the loop, so all "weight" below will be
> the same value.  To keep it simple, I think we can just copy weights
> from the global iw_table and consider the default value?
> 

another rebase error here from my tested code, this should have been
node = prev_node;
while (...)
    node = next_node_in(node, nodes);

I can change it to a for loop as suggested, but for more info on why I
did it this way, see the chunk below

> > +		} else if (!delta_depleted) {
> > +			/* if there was no delta, track last allocated node */
> > +			resume_node = node;
> > +			resume_weight = i < (nnodes - 1) ? weights[i+1] :
> > +							   weights[0];
                        ^ this line acquires the weight of the *NEXT* node
			  another chunk prior to this does the same
			  thing.  I suppose i can use next_node_in()
			  instead and just copy the entire weigh array
			  though, if that is preferable.
> > +		}
> 
> Can the above code be simplified as something like below?
> 
>         resume_node = prev_node;
>         resume_weight = 0;
>         for (...) {
>                 ...
>                 if (delta > weight) {
> 			node_pages += weight;
> 			delta -= weight;
> 		} else if (delta) {
> 			node_pages += delta;
>         		/* if delta depleted, resume from this node */
>                         if (delta < weight) {
>                                 resume_node = prev_node;
>                                 resume_weight = weight - delta;
>                         } else {
>                                 resume_node = node;
>                         }
> 			delta = 0;
>                 }
>                 ...
>         }
> 

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.

If it's NUMA_NO_NODE, then the logic you have above will say "the next
node has no remaining weights assigned" and skip it on the next call to
weighted_interleave_nid or weighted_interleave_nodes.

This is incorrect - we want the weight of the first node to be
resume_weight, which is what this chunk does:

if (delta >= weight) {
    /* if delta == weight, get next node weight */
    resume_weight = i < (nnodes - 1) ? weights[i+1] : weights[0];
else if (delta) { /* delta < weight */
    /* there's a remaining weight, use the that for resume weight */
    resume_weight = weight - (node_pages % weight);
} else if (!delta_depleted) {
    /* there was never a delta, track the last node and get the weight
     * of the node AFTER that node, that's the resume weight */
    resume_weight = i < (nnodes - 1) ? weights[i+1] : weights[0];
}

If il_prev is an actual node, and delta == 0, we want to return with
(il_prev = prev_node) but with the weight set to the weight of the
first node we're about to allocate from.

This is the reason for the annoying logic here: We have to come out of
this loop with the actual node and the actual weight.

I'll try to clean it up further and get my test suite to pass.

~Gregory