linux-kernel - Re: [PATCH] mm/page-writeback: Update the comment of Dirty position control

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Message-ID: <20210511090632.GH24154@quack2.suse.cz>
Date:   Tue, 11 May 2021 11:06:32 +0200
From:   Jan Kara <jack@...e.cz>
To:     Chi Wu <wuchi.zero@...il.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, tj@...nel.org, mszeredi@...hat.com,
        sedat.dilek@...il.com, axboe@...com, jack@...e.cz,
        hcochran@...nelspring.com
Subject: Re: [PATCH] mm/page-writeback: Update the comment of Dirty position
 control

On Tue 11-05-21 13:26:26, Chi Wu wrote:
> As the value of pos_ratio_polynom() clamp between 0 and
> 2LL << RATELIMIT_CALC_SHIFT, the global control line should be
> consistent with it.
> 
> Signed-off-by: Chi Wu <wuchi.zero@...il.com>
> ---
>  mm/page-writeback.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 0062d5c57d41..e9a4f6cf12bb 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -845,7 +845,7 @@ static long long pos_ratio_polynom(unsigned long setpoint,
>   *     ^ pos_ratio
>   *     |
>   *     |            |<===== global dirty control scope ======>|
> - * 2.0 .............*
> + * 2.0 * * * * * * *
>   *     |            .*
>   *     |            . *
>   *     |            .   *

Generally, pos_ratio is used only within [freerun, limit] range so its
values outside of that range don't matter much. But you're right that
pos_ratio_polynom() clamps the computed value as you describe and we can
visualize that in the graph as well. Just please move the '*' line you add
by one space to the right so that the curve really follows the cubic curve
upto 'freerun' mark (the vertical '.' line).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR