linux-kernel - Re: [PATCH v3 4/4] mm/mempolicy: change cur_il_weight to atomic and carry the node with it

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Date: Mon, 29 Jan 2024 10:48:47 -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
Subject: Re: [PATCH v3 4/4] mm/mempolicy: change cur_il_weight to atomic and
 carry the node with it

On Mon, Jan 29, 2024 at 04:17:46PM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@...verge.com> writes:
> 
> > Using current->il_prev between these two policies, is just plain incorrect,
> > so I will need to rethink this, and the existing code will need to be
> > updated such that weighted_interleave does not use current->il_prev.
> 
> IIUC, weighted_interleave_nodes() is only used for mempolicy of tasks
> (set_mempolicy()), as in the following code.
> 
> +		*nid = (ilx == NO_INTERLEAVE_INDEX) ?
> +			weighted_interleave_nodes(pol) :
> +			weighted_interleave_nid(pol, ilx);
>

Was digging through this the past couple of days.  It does look like
this is true - because if (pol) comes from a vma, ilx will not be
NO_INTERLEAVE_INDEX.  If this changes in the future, however,
weighted_interleave_nodes may begin to miscount under heavy contention.

It may be worth documenting this explicitly, because this is incredibly
non-obvious.  I will add a comment to this chunk here.

> But, in contrast, it's bad to put task-local "current weight" in
> mempolicy.  So, I think that it's better to move cur_il_weight to
> task_struct.  And maybe combine it with current->il_prev.
> 

Given all of this, I think is reasonably. That is effectively what is
happening anyway for anyone that just uses `numactl -w --interleave=...`

Style question: is it preferable add an anonymous union into task_struct:

union {
    short il_prev;
    atomic_t wil_node_weight;
};

Or should I break out that union explicitly in mempolicy.h?

The latter involves additional code updates in mempolicy.c for the union
name (current->___.il_prev) but it lets us add documentation to mempolicy.h

~Gregory