Re: [PATCH] mm/damon/core: fix wasteful CPU calls by skipping non-existent targets

[SeongJae Park <sj@kernel.org>]

On Wed, 10 Dec 2025 13:25:08 +0800 Enze Li <lienze@kylinos.cn> wrote:

> Currently, DAMON does not proactively clean up invalid monitoring
> targets during its runtime.  When some monitored processes exit, DAMON
> continues to make the following unnecessary function calls,
> 
>   --damon_for_each_target--
>   --damon_for_each_region--
>       damon_do_apply_schemes
>         damos_apply_scheme
>           damon_va_apply_scheme
>             damos_madvise
>               damon_get_mm
> 
> it is only in the damon_get_mm() function that it may finally discover
> the target no longer exists, which wastes CPU resources. A simple idea
> is to check for the existence of monitoring targets within the
> kdamond_need_stop() function and promptly clean up non-existent targets.
> 
> However, SJ pointed out that this approach is problematic because the
> online commit logic incorrectly uses list indices to update the
> monitoring state.  This can lead to data loss if the target list is
> changed concurrently.  Meanwhile, SJ suggests checking for target
> existence at the damon_for_each_target level, and if a target does not
> exist, simply skip it and proceed to the next one.

Thank you for making this patch, Enze!

> 
> Suggested-by: SeongJae Park <sj@kernel.org>
> Signed-off-by: Enze Li <lienze@kylinos.cn>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

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