linux-kernel - Re: [PATCH mm-unstable] mm/damon/core: increase regions merge aggressiveness while respecting min_nr

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Date: Wed, 26 Jun 2024 15:05:16 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: SeongJae Park <sj@...nel.org>
Cc: damon@...ts.linux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable] mm/damon/core: increase regions merge
 aggressiveness while respecting min_nr_regions

On Wed, 26 Jun 2024 14:49:54 -0700 SeongJae Park <sj@...nel.org> wrote:

> > @@ -1716,8 +1717,8 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
> >  			nr_regions += damon_nr_regions(t);
> >  		}
> >  		threshold = max(1, threshold * 2);
> > -		sz_limit = max(1, sz_limit * 2);
> > -	} while (nr_regions > c->attrs.max_nr_regions);
> > +	} while (nr_regions > c->attrs.max_nr_regions &&
> > +			threshold <= max_thres);
> 
> This code means that kdamond_merge_regions() stops this repeated merge attempt
> if the merge threshold that increased for next attempt is higher than the
> possible maximum threshold.  And because the increase of the threshold is made
> by picking a maximum value between one and the last-used threshold multiplying
> two, the merge attempt with maximum threshold will not be made unless both the
> maximum threshold and the threshold to increase are powers of two.  In maximum
> situation (e.g., region 1 has 100% access frequency, region 2 has 0% access
> frequency, so on), this means the max_nr_regions violation cannot be recovered
> by the attempts.
> 
> This can be fixed by changing it to stop repeated attempt if the last-used
> threshold is same to or higher than the maximum possible threshold, like below.
> 
> I'll send the fix of the fix as a formal patch soon.
> 
> FYI, the original fix is definitely better to be merged in stable kernels, but
> not urgent in my opinion, since the problematic case is not common and the
> behavior was same since the beginning of DAMON.  Andrew, if you feel the
> original fix is not stable yet, please feel free to delay moving it to
> hotfix-stable for one week or two.

That's fine - we can merge cc:stable patches any time, really - they
will still get backported.  There's only a hurry to get fixes merged up
if they're security-related or if the issue is causing people problems.

In this case I'll await your -fix-2.patch and we can merge the patch
next week.