linux-kernel - Re: [RFC PATCH v2 2/2] mm/damon/paddr: Allow multiple migrate targets

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Message-ID: <CAMvvPS7ozBjY-YWHrkkGVQA8ug=0=xT=BeyZxi0YrHp6PNQmHg@mail.gmail.com>
Date: Mon, 23 Jun 2025 09:27:16 -0500
From: Bijan Tabatabai <bijan311@...il.com>
To: SeongJae Park <sj@...nel.org>
Cc: damon@...ts.linux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com, 
	matthew.brost@...el.com, joshua.hahnjy@...il.com, rakie.kim@...com, 
	byungchul@...com, gourry@...rry.net, ying.huang@...ux.alibaba.com, 
	apopple@...dia.com, bijantabatab@...ron.com, venkataravis@...ron.com, 
	emirakhur@...ron.com, ajayjoshi@...ron.com, vtavarespetr@...ron.com
Subject: Re: [RFC PATCH v2 2/2] mm/damon/paddr: Allow multiple migrate targets

On Sat, Jun 21, 2025 at 1:11 PM SeongJae Park <sj@...nel.org> wrote:
>
> On Sat, 21 Jun 2025 11:02:15 -0700 SeongJae Park <sj@...nel.org> wrote:
>
> [...]
> > I'd hence suggest to implement and use a simple weights handling mechanism
> > here.  It could be roud-robin way, like weighted interleaving, or probabilistic
> > way, using damon_rand().
> >
> > The round-robin way may be simpler in my opinion.  For example,
> >
> > unsigned int damos_pa_nid_to_migrate(struct damos_migrate_dest *dest)
> > {
> >       static unsigned int nr_migrated = 0;
> >       unsigned int total_weight = 0;
> >       unsigned int weights_to_ignore;
> >       size_t i;
> >
> >       for (i = 0; i < dest->nr_dests; i++)
> >               total_weight += dest->weight_arr[i];
> >       weights_to_ignore = nr_migrate++ % total_weight;
>
> Actually, probabilistic way may be not that complicated.  Maybe we could to
> below here.
>
>         return damon_rand(0, total_weight) >= weight_to_ignore;
>
> >       total_weight = 0;
> >       for (i = 0; i < dest->nr_dests; i++) {
> >               total_weight += dest->weight_arr[i];
> >               if (total_weight >= weights_to_ignore)
> >                       return dest->node_id_arr[i];
> >       }
> >       WARN_ON_ONCE(1, "I don't know what I did wrong");
> >       return 0;
> > }
>
> But damon_rand() might be more expensive than the roud-robin way, and arguably
> roud-robin way is what usrs who familiar with weighted interleaving may easily
> expect and even prefer?  I have no preferrence here.

I think my comments about the round-robin approach also apply here,
but all things equal, I generally prefer being deterministic over
being probabilistic when possible.

Bijan

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