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Date:   Sat, 1 Aug 2020 02:49:12 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Alex Zhuravlev <azhuravlev@...mcloud.com>,
        Artem Blagodarenko <artem.blagodarenko@...il.com>
Subject: Re: [PATCH 2/4] ext4: skip non-loaded groups at cr=0/1 when scanning
 for good groups


> On Jul 31, 2020, at 1:08 PM, Theodore Ts'o <tytso@....edu> wrote:
> 
> From: Alex Zhuravlev <azhuravlev@...mcloud.com>
> 
> cr=0 is supposed to be an optimization to save CPU cycles, but if
> buddy data (in memory) is not initialized then all this makes no sense
> as we have to do sync IO taking a lot of cycles.  Also, at cr=0
> mballoc doesn't choose any avaibale chunk.  cr=1 also skips groups

(typo) "available"

> using heuristic based on avg. fragment size.  It's more useful to skip
> such groups and switch to cr=2 where groups will be scanned for
> available chunks.  However, we always read the first block group in a
> flex_bg so metadata blocks will get read into the first flex_bg if
> possible.
> 
> Using sparse image and dm-slow virtual device of 120TB was
> simulated, then the image was formatted and filled using debugfs to
> mark ~85% of available space as busy.  mount process w/o the patch
> couldn't complete in half an hour (according to vmstat it would take
> ~10-11 hours).  With the patch applied mount took ~20 seconds.
> 
> Lustre-bug-id: https://jira.whamcloud.com/browse/LU-12988
> Signed-off-by: Alex Zhuravlev <azhuravlev@...mcloud.com>
> Reviewed-by: Andreas Dilger <adilger@...mcloud.com>
> Reviewed-by: Artem Blagodarenko <artem.blagodarenko@...il.com>

Looks good, one trivial cleanup below could also be done before landing.

> ---
> fs/ext4/mballoc.c | 22 +++++++++++++++++++++-
> 1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index fcc702f1ff15..b1ef35a9e9f1 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2195,7 +2196,26 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
> 
> 	/* We only do this if the grp has never been initialized */
> 	if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
> -		ret = ext4_mb_init_group(ac->ac_sb, group, GFP_NOFS);
> +		struct ext4_group_desc *gdp =
> +			ext4_get_group_desc(sb, group, NULL);
> +		int ret;
> +
> +		/* cr=0/1 is a very optimistic search to find large
> +		 * good chunks almost for free.  If buddy data is not
> +		 * ready, then this optimization makes no sense.  But
> +		 * we never skip the first block group in a flex_bg,
> +		 * since this gets used for metadata block allocation,
> +		 * and we want to make sure we locate metadata blocks
> +		 * in the first block group in the flex_bg if
> +		 * possible.

(style) "possible" could fit on the previous line?

> +		 */
> +		if (cr < 2 &&
> +		    (!sbi->s_log_groups_per_flex ||
> +		     ((group & ((1 << sbi->s_log_groups_per_flex) - 1)) != 0)) &&
> +		    !(ext4_has_group_desc_csum(sb) &&
> +		      (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))))
> +			return 0;
> +		ret = ext4_mb_init_group(sb, group, GFP_NOFS);
> 		if (ret)
> 			return ret;
> 	}
> --
> 2.24.1
> 


Cheers, Andreas






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