linux-ext4 - Re: [RFC 07/11] ext4: Avoid scanning smaller extents in BG during CR1

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Date:   Thu, 9 Mar 2023 13:20:52 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc:     linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>, Ritesh Harjani <ritesh.list@...il.com>
Subject: Re: [RFC 07/11] ext4: Avoid scanning smaller extents in BG during CR1

On Fri 27-01-23 18:07:34, Ojaswin Mujoo wrote:
> When we are inside ext4_mb_complex_scan_group() in CR1, we can be sure
> that this group has atleast 1 big enough continuous free extent to satisfy
> our request because (free / fragments) > goal length.
> 
> Hence, instead of wasting time looping over smaller free extents, only
> try to consider the free extent if we are sure that it has enough
> continuous free space to satisfy goal length. This is particularly
> useful when scanning highly fragmented BGs in CR1 as, without this
> patch, the allocator might stop scanning early before reaching the big
> enough free extent (due to ac_found > mb_max_to_scan) which causes us to
> uncessarily trim the request.
> 
> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>

Looks good to me. Feel free to add:

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

								Honza

> ---
>  fs/ext4/mballoc.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index c4ab8f412d32..14529d2fe65f 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2279,7 +2279,7 @@ void ext4_mb_complex_scan_group(struct ext4_allocation_context *ac,
>  	struct super_block *sb = ac->ac_sb;
>  	void *bitmap = e4b->bd_bitmap;
>  	struct ext4_free_extent ex;
> -	int i;
> +	int i, j, freelen;
>  	int free;
>  
>  	free = e4b->bd_info->bb_free;
> @@ -2306,6 +2306,23 @@ void ext4_mb_complex_scan_group(struct ext4_allocation_context *ac,
>  			break;
>  		}
>  
> +		if (ac->ac_criteria < CR2) {
> +			/*
> +			 * In CR1, we are sure that this group will
> +			 * have a large enough continuous free extent, so skip
> +			 * over the smaller free extents
> +			 */
> +			j = mb_find_next_bit(bitmap,
> +						EXT4_CLUSTERS_PER_GROUP(sb), i);
> +			freelen = j - i;
> +
> +			if (freelen < ac->ac_g_ex.fe_len) {
> +				i = j;
> +				free -= freelen;
> +				continue;
> +			}
> +		}
> +
>  		mb_find_extent(e4b, i, ac->ac_g_ex.fe_len, &ex);
>  		if (WARN_ON(ex.fe_len <= 0))
>  			break;
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR