Re: [RFC PATCH 02/14] mm/damon/core: introduce repeat mode damon_call()

[SeongJae Park <sj@kernel.org>]

On Sun,  6 Jul 2025 13:00:06 -0700 SeongJae Park <sj@kernel.org> wrote:

> damon_call() can be useful for reading or writing DAMON internal data
> for one time.  A common pattern of DAMON core usage from DAMON modules
> is doing such reads and writes repeatedly, for example, to periodically
> update the DAMOS stats.  To do that with damon_call(), callers should
> call damon_call() repeatedly, with their own delay loop.  Each caller
> doing that is repetitive.  Introduce a repeat mode damon_call().
> Callers can use the mode by setting a new field in damon_call_control.
> If the mode is turned on, damon_call() returns success immediately, and
> DAMON repeats invoking the callback function inside the kdamond main
> loop.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
[...]
> @@ -2389,6 +2393,7 @@ static void kdamond_usleep(unsigned long usecs)
>  static void kdamond_call(struct damon_ctx *ctx, bool cancel)
>  {
>  	struct damon_call_control *control;
> +	LIST_HEAD(repeat_controls);
>  	int ret = 0;
>  
>  	while (true) {
> @@ -2407,8 +2412,18 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
>  		mutex_lock(&ctx->call_controls_lock);
>  		list_del(&control->list);
>  		mutex_unlock(&ctx->call_controls_lock);
> -		complete(&control->completion);
> +		if (!control->repeat)
> +			complete(&control->completion);
> +		else
> +			list_add(&control->list, &repeat_controls);
>  	}
> +	control = list_first_entry_or_null(&repeat_controls,
> +			struct damon_call_control, list);
> +	if (!control || cancel)
> +		return;
> +	mutex_lock(&ctx->call_controls_lock);
> +	list_add_tail(&control->list, &ctx->call_controls);
> +	mutex_unlock(&ctx->call_controls_lock);
>  }

Below fixup should be added on this.  Without it, the repeat_controls handling
is never executed, so callback will be invoked only once.

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 7a4dc76dd023..3c0c3cef43e7 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2402,7 +2402,7 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
                                struct damon_call_control, list);
                mutex_unlock(&ctx->call_controls_lock);
                if (!control)
-                       return;
+                       break;
                if (cancel) {
                        control->canceled = true;
                } else {

I'll add this on next version of this series.


Thanks,
SJ

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