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From:   sj@kernel.org
To:     Jonghyeon Kim <tome01@ajou.ac.kr>
Cc:     SeongJae Park <sj@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/damon: prevent activated scheme from sleeping by deactivated schemes
Date:   Wed, 30 Mar 2022 11:55:16 +0000
Message-Id: <20220330115516.32126-1-sj@kernel.org>
In-Reply-To: <20220330105302.32114-1-tome01@ajou.ac.kr>
Precedence: bulk

Hi Jonghyeon,

On Wed, 30 Mar 2022 19:53:02 +0900 Jonghyeon Kim <tome01@ajou.ac.kr> wrote:

> In the DAMON, the minimum wait time of the schemes decides whether the
> kernel wakes up 'kdamon_fn()'. But since the minimum wait time is
> initialized to zero, there are corner cases against the original objective.
> 
> For example, if we have several schemes for one target, and if the wait
> time of the first scheme is zero, the minimum wait time will set zero,
> which means 'kdamond_fn()' should wake up to apply this scheme. However,
> in the following scheme, wait time can be set to non-zero. Thus, the
> mininum wait time will be set to non-zero, which can cause sleeping this
> interval for 'kdamon_fn()' due to one deactivated last scheme.

Good catch, thanks!

> 
> This commit prevents making DAMON monitoring inactive state due to other
> deactivated schemes.
> 
> Signed-off-by: Jonghyeon Kim <tome01@ajou.ac.kr>

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


Thanks,
SJ

> ---
>  mm/damon/core.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index c1e0fed4e877..5ce8d7c867f0 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1019,12 +1019,15 @@ static int kdamond_wait_activation(struct damon_ctx *ctx)
>  	struct damos *s;
>  	unsigned long wait_time;
>  	unsigned long min_wait_time = 0;
> +	bool init_wait_time = false;
>  
>  	while (!kdamond_need_stop(ctx)) {
>  		damon_for_each_scheme(s, ctx) {
>  			wait_time = damos_wmark_wait_us(s);
> -			if (!min_wait_time || wait_time < min_wait_time)
> +			if (!init_wait_time || wait_time < min_wait_time) {
> +				init_wait_time = true;
>  				min_wait_time = wait_time;
> +			}
>  		}
>  		if (!min_wait_time)
>  			return 0;
> -- 
> 2.17.1
>