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

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Message-Id: <20250621181127.36394-1-sj@kernel.org>
Date: Sat, 21 Jun 2025 11:11:27 -0700
From: SeongJae Park <sj@...nel.org>
To: SeongJae Park <sj@...nel.org>
Cc: Bijan Tabatabai <bijan311@...il.com>,
	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,
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	ying.huang@...ux.alibaba.com,
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	venkataravis@...ron.com,
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Subject: Re: [RFC PATCH v2 2/2] mm/damon/paddr: Allow multiple migrate targets

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.


Thanks,
SJ

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