Re: [RFC PATCH v2 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park <sj@kernel.org>]

On Wed, 18 Mar 2026 22:21:44 -0700 SeongJae Park <sj@kernel.org> wrote:

> DAMON supports only start and stop of the execution.  When it is
> stopped, its internal data that it self-trained goes away.  It will be
> useful if the execution can be paused and resumed with the previous
> self-trained data.
> 
> Introduce per-context API parameter, 'paused', for the purpose.  The
> parameter can be set and unset while DAMON is running and paused, using
> the online parameters commit helper functions (damon_commit_ctx() and
> damon_call()).  Once 'paused' is set, the kdamond_fn() main loop does
> only limited works with sampling interval sleep during the works.  The
> limited works include the handling of the online parameters update, so
> that users can unset the 'pause' and resume the execution when they
> want.  It also keep checking DAMON stop conditions and handling of it,
> so that DAMON can be stopped while paused if needed.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  include/linux/damon.h | 2 ++
>  mm/damon/core.c       | 9 +++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index 3a441fbca170d..421e51eff3bd2 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -811,6 +811,8 @@ struct damon_ctx {
>  	 * intervals tuning
>  	 */
>  	unsigned long next_intervals_tune_sis;
> +	/* pause kdamond main loop */
> +	bool pause;
>  	/* for waiting until the execution of the kdamond_fn is started */
>  	struct completion kdamond_started;
>  	/* for scheme quotas prioritization */
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 339325e1096bc..0b6cb63d64d0e 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1331,6 +1331,7 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
>  		if (err)
>  			return err;
>  	}
> +	dst->pause = src->pause;
>  	dst->ops = src->ops;
>  	dst->addr_unit = src->addr_unit;
>  	dst->min_region_sz = src->min_region_sz;
> @@ -2978,6 +2979,14 @@ static int kdamond_fn(void *data)
>  		 * kdamond_merge_regions() if possible, to reduce overhead
>  		 */
>  		kdamond_call(ctx, false);
> +		while (ctx->pause) {
> +			if (kdamond_need_stop(ctx))
> +				goto done;
> +			kdamond_usleep(ctx->attrs.sample_interval);
> +			/* allow caller unset pause via damon_call() */
> +			kdamond_call(ctx, false);
> +			damos_walk_cancel(ctx);

Sashiko comment
(https://sashiko.dev/#/patchset/20260319052157.99433-2-sj@kernel.org) and my
reply below.

: Can this cause spurious cancellations of DAMOS walk requests if the context
: is unpaused?
: 
: If kdamond_call() processes a commit that sets ctx->pause = false, and a new
: walk request is queued concurrently or immediately after, this unconditional
: call to damos_walk_cancel() will cancel the newly submitted request before
: the loop condition is evaluated again to exit the loop.
: 
: Would it be safer to verify if the context is still paused before cancelling,
: perhaps by checking if (ctx->pause) before calling damos_walk_cancel()?

Yes, it can cause unnecessary cancel of damos_walk() request.  But cancelling a
few more damos_walk() request is no big deal.  The caller and user can show it
is cancelled, and just ask it again.

So, not a problem.


Thanks,
SJ

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