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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 22 Jun 2021 07:59:35 -0700
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Subject: Re: [PATCH v31 03/13] mm/damon: Adaptively adjust regions
To:     SeongJae Park <sj38.park@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        SeongJae Park <sjpark@amazon.de>, Jonathan.Cameron@huawei.com,
        acme@kernel.org, alexander.shishkin@linux.intel.com,
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        David Hildenbrand <david@redhat.com>, dwmw@amazon.com,
        Marco Elver <elver@google.com>, "Du, Fan" <fan.du@intel.com>,
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        Vlastimil Babka <vbabka@suse.cz>,
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On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@gmail.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.de>
>
> Even somehow the initial monitoring target regions are well constructed
> to fulfill the assumption (pages in same region have similar access
> frequencies), the data access pattern can be dynamically changed.  This
> will result in low monitoring quality.  To keep the assumption as much
> as possible, DAMON adaptively merges and splits each region based on
> their access frequency.
>
> For each ``aggregation interval``, it compares the access frequencies of
> adjacent regions and merges those if the frequency difference is small.
> Then, after it reports and clears the aggregated access frequency of
> each region, it splits each region into two or three regions if the
> total number of regions will not exceed the user-specified maximum
> number of regions after the split.
>
> In this way, DAMON provides its best-effort quality and minimal overhead
> while keeping the upper-bound overhead that users set.
>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> Reviewed-by: Leonard Foerster <foersleo@amazon.de>
> Reviewed-by: Fernand Sieber <sieberf@amazon.com>
[...]
>
> +unsigned int damon_nr_regions(struct damon_target *t)
> +{
> +       struct damon_region *r;
> +       unsigned int nr_regions = 0;
> +
> +       damon_for_each_region(r, t)
> +               nr_regions++;

This bugs me everytime. Please just have nr_regions field in the
damon_target instead of traversing the list to count the number of
regions.

Other than that, it looks good to me.