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Date: Thu, 4 Apr 2024 15:22:21 +0200
From: Jan Kara <jack@suse.cz>
To: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: tytso@mit.edu, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org,
	linux-kernel@vger.kernel.org, jack@suse.cz, ojaswin@linux.ibm.com,
	ritesh.list@gmail.com
Subject: Re: [PATCH 1/5] ext4: keep "prefetch_grp" and "nr" consistent
Message-ID: <20240404132221.uliif3mw7acy7t7l@quack3>
References: <20240326213823.528302-1-shikemeng@huaweicloud.com>
 <20240326213823.528302-2-shikemeng@huaweicloud.com>
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In-Reply-To: <20240326213823.528302-2-shikemeng@huaweicloud.com>

On Wed 27-03-24 05:38:19, Kemeng Shi wrote:
> Keep "prefetch_grp" and "nr" consistent to avoid to call
> ext4_mb_prefetch_fini with non-prefetched groups.
> When we step into next criteria, "prefetch_grp" is set to prefetch start
> of new criteria while "nr" is number of the prefetched group in previous
> criteria. If previous criteria and next criteria are both inexpensive
> (< CR_GOAL_LEN_SLOW) and prefetch_ios reachs sbi->s_mb_prefetch_limit
> in previous criteria, "prefetch_grp" and "nr" will be inconsistent and
> may introduce unexpected cost to do ext4_mb_init_group for non-prefetched
> groups.
> Reset "nr" to 0 when we reset "prefetch_grp" to start of prefech in next
> criteria to ensure "prefetch_grp" and "nr" are consistent.
> 
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/ext4/mballoc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 12b3f196010b..a61fc52956b2 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2856,6 +2856,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  		group = ac->ac_g_ex.fe_group;
>  		ac->ac_groups_linear_remaining = sbi->s_mb_max_linear_groups;
>  		prefetch_grp = group;
> +		nr = 0;
>  
>  		for (i = 0, new_cr = cr; i < ngroups; i++,
>  		     ext4_mb_choose_next_group(ac, &new_cr, &group, ngroups)) {
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR