Re: (sashiko) [PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260321181343.93971-2-sj@xxxxxxxxxx
# start of sashiko.dev inline review
commit b56d17c42c1dc6d883f3a75d785a05ba53b5c35b
Author: SeongJae Park <sj@xxxxxxxxxx>

mm/damon/core: introduce damon_ctx->paused

This commit introduces a paused parameter to the DAMON context API.
It allows users to pause and resume kdamond execution while preserving
self-trained data. The main loop is updated to perform limited work
and sleep when the context is paused.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index db6c67e52d2b..ce010009ca13 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -3003,6 +3004,14 @@ static int kdamond_fn(void *data)
>  		kdamond_call(ctx, false);
>  		if (ctx->maybe_corrupted)
>  			break;
> +		while (ctx->pause) {
> +			if (kdamond_need_stop(ctx))
> +				goto done;
> +			kdamond_usleep(ctx->attrs.sample_interval);

This isn't a bug, but polling with kdamond_usleep() using the sample
interval could cause many wakeups per second even while the context is
paused. Would it be more efficient to use a waitqueue here to consume
near-zero CPU until the thread is explicitly woken?

> +			/* allow caller unset pause via damon_call() */
> +			kdamond_call(ctx, false);

If a parameter update fails during kdamond_call() and sets
ctx->maybe_corrupted to true, the loop will continue to run or exit the
pause loop and proceed to kdamond_apply_schemes() with corrupted
structures.

Is it possible to bypass the corruption safeguard here? Should we check
ctx->maybe_corrupted inside this loop to safely terminate the thread?

> +			damos_walk_cancel(ctx);

If kdamond_call() processes a request that unpauses the context, this
will immediately cancel any pending or concurrent damos_walk() requests
before the loop condition is re-evaluated.

Could this cause spurious cancellations of walk requests when unpausing
the context?

> +		}
>  		if (!list_empty(&ctx->schemes))
>  			kdamond_apply_schemes(ctx);
>  		else


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