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From:   SeongJae Park <sj@kernel.org>
To:     Jonghyeon Kim <tome01@ajou.ac.kr>
Cc:     akpm@linux-foundation.org, Jonathan.Cameron@Huawei.com,
        amit@kernel.org, benh@kernel.crashing.org, corbet@lwn.net,
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        rientjes@google.com, shakeelb@google.com, shuah@kernel.org,
        linux-damon@amazon.com, linux-mm@kvack.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v1 2/3] mm/damon/core: Add damon_start_one()
Date:   Tue, 22 Feb 2022 09:53:28 +0000
Message-Id: <20220222095328.7962-1-sj@kernel.org>
In-Reply-To: <20220218102611.31895-3-tome01@ajou.ac.kr>
Precedence: bulk

Hello Jonghyeon,

On Fri, 18 Feb 2022 19:26:10 +0900 Jonghyeon Kim <tome01@ajou.ac.kr> wrote:

> damon_start() function is designed to start multiple damon monitoring
> contexts. But, sometimes we need to start monitoring one context.
> Although __damon_start() could be considered to start for one monitoring
> context, it seems reasonable to adopt a new function that does not need
> to handle 'damon_lock' from the caller.
> 
> Signed-off-by: Jonghyeon Kim <tome01@ajou.ac.kr>
> ---
>  include/linux/damon.h |  1 +
>  mm/damon/core.c       | 25 +++++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index c0adf1566603..069577477662 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -511,6 +511,7 @@ int damon_register_ops(struct damon_operations *ops);
>  int damon_select_ops(struct damon_ctx *ctx, enum damon_ops_id id);
>  
>  int damon_start(struct damon_ctx **ctxs, int nr_ctxs);
> +int damon_start_one(struct damon_ctx *ctx);
>  int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
>  
>  #endif	/* CONFIG_DAMON */
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 290c9c0535ee..e43f138a3489 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -466,6 +466,31 @@ int damon_start(struct damon_ctx **ctxs, int nr_ctxs)
>  	return err;
>  }
>  
> +/**
> + * damon_start_one() - Starts the monitorings for one context.
> + * @ctx:	monitoring context
> + *
> + * This function starts one monitoring thread for only one monitoring context
> + * handling damon_lock.
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +int damon_start_one(struct damon_ctx *ctx)
> +{
> +	int err = 0;
> +
> +	mutex_lock(&damon_lock);
> +	err = __damon_start(ctx);
> +	if (err) {
> +		mutex_unlock(&damon_lock);
> +		return err;
> +	}
> +	nr_running_ctxs++;
> +	mutex_unlock(&damon_lock);
> +
> +	return err;
> +}
> +

IMHO, this looks like an unnecessary duplication of code.  Unless this is
needed for a real usecase, this change might unnecessarily make the code only a
little bit more complicated.  And to my understanding of the next patch, this
is not really needed for this patchset.  I will left comments on the patch.  If
I'm missing something, please clarify why this is really needed.

Furthermore, damon_start() starts a set of DAMON contexts in exclusive manner,
to ensure there will be no interference.  This patch breaks the assumption.
That is, contexts that started with damon_start() could be interfered by other
contexts that started with damon_start_one().  I have a plan to make
damon_start() also work for non-exclusive contexts group[1], though.

[1] https://lore.kernel.org/linux-mm/20220217161938.8874-3-sj@kernel.org/


Thanks,
SJ

>  /*
>   * __damon_stop() - Stops monitoring of given context.
>   * @ctx:	monitoring context
> -- 
> 2.17.1
>