linux-kernel - Re: [PATCH v3 02/17] ext4: separate stream goal hits from s_bal

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Message-ID: <aHjQmHgSYmjwI6g8@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Thu, 17 Jul 2025 15:59:44 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
        jack@...e.cz, linux-kernel@...r.kernel.org, julia.lawall@...ia.fr,
        yi.zhang@...wei.com, yangerkun@...wei.com, libaokun@...weicloud.com
Subject: Re: [PATCH v3 02/17] ext4: separate stream goal hits from
 s_bal_goals for better tracking

On Mon, Jul 14, 2025 at 09:03:12PM +0800, Baokun Li wrote:
> In ext4_mb_regular_allocator(), after the call to ext4_mb_find_by_goal()
> fails to achieve the inode goal, allocation continues with the stream
> allocation global goal. Currently, hits for both are combined in
> sbi->s_bal_goals, hindering accurate optimization.
> 
> This commit separates global goal hits into sbi->s_bal_stream_goals. Since
> stream allocation doesn't use ac->ac_g_ex.fe_start, set fe_start to -1.
> This prevents stream allocations from being counted in s_bal_goals. Also
> clear EXT4_MB_HINT_TRY_GOAL to avoid calling ext4_mb_find_by_goal again.
> 
> After adding `stream_goal_hits`, `/proc/fs/ext4/sdx/mb_stats` will show:
> 
> mballoc:
> 	reqs: 840347
> 	success: 750992
> 	groups_scanned: 1230506
> 	cr_p2_aligned_stats:
> 		hits: 21531
> 		groups_considered: 411664
> 		extents_scanned: 21531
> 		useless_loops: 0
> 		bad_suggestions: 6
> 	cr_goal_fast_stats:
> 		hits: 111222
> 		groups_considered: 1806728
> 		extents_scanned: 467908
> 		useless_loops: 0
> 		bad_suggestions: 13
> 	cr_best_avail_stats:
> 		hits: 36267
> 		groups_considered: 1817631
> 		extents_scanned: 156143
> 		useless_loops: 0
> 		bad_suggestions: 204
> 	cr_goal_slow_stats:
> 		hits: 106396
> 		groups_considered: 5671710
> 		extents_scanned: 22540056
> 		useless_loops: 123747
> 	cr_any_free_stats:
> 		hits: 138071
> 		groups_considered: 724692
> 		extents_scanned: 23615593
> 		useless_loops: 585
> 	extents_scanned: 46804261
> 		goal_hits: 1307
> 		stream_goal_hits: 236317
> 		len_goal_hits: 155549
> 		2^n_hits: 21531
> 		breaks: 225096
> 		lost: 35062
> 	buddies_generated: 40/40
> 	buddies_time_used: 48004
> 	preallocated: 5962467
> 	discarded: 4847560
> 
> Signed-off-by: Baokun Li <libaokun1@...wei.com>
> ---
>  fs/ext4/ext4.h    |  1 +
>  fs/ext4/mballoc.c | 11 +++++++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 9df74123e7e6..8750ace12935 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1646,6 +1646,7 @@ struct ext4_sb_info {
>  	atomic_t s_bal_cX_ex_scanned[EXT4_MB_NUM_CRS];	/* total extents scanned */
>  	atomic_t s_bal_groups_scanned;	/* number of groups scanned */
>  	atomic_t s_bal_goals;	/* goal hits */
> +	atomic_t s_bal_stream_goals;	/* stream allocation global goal hits */
>  	atomic_t s_bal_len_goals;	/* len goal hits */
>  	atomic_t s_bal_breaks;	/* too long searches */
>  	atomic_t s_bal_2orders;	/* 2^order hits */
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 336d65c4f6a2..f56ac477c464 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2849,8 +2849,9 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  		/* TBD: may be hot point */
>  		spin_lock(&sbi->s_md_lock);
>  		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
>  		spin_unlock(&sbi->s_md_lock);
> +		ac->ac_g_ex.fe_start = -1;
> +		ac->ac_flags &= ~EXT4_MB_HINT_TRY_GOAL;

Hey Baokun, I was a bit late to review this in v2 so I'll add the
comment here:

So this is mostly to account for retires right? Maybe rather than
disabling goal allocation a better way to do this is resetting the
original goal group and goal start in the retry logic of
ext4_mb_new_blocks()? Since we drop preallocations before retrying, this
way we might actually find our goal during the retry. Its a slim chance
though but still feels like the right way to do it.

Thoughts?

Regards,
ojaswin

>  	}
>  
>  	/*
> @@ -3000,8 +3001,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  		}
>  	}
>  
> -	if (sbi->s_mb_stats && ac->ac_status == AC_STATUS_FOUND)
> +	if (sbi->s_mb_stats && ac->ac_status == AC_STATUS_FOUND) {
>  		atomic64_inc(&sbi->s_bal_cX_hits[ac->ac_criteria]);
> +		if (ac->ac_flags & EXT4_MB_STREAM_ALLOC &&
> +		    ac->ac_b_ex.fe_group == ac->ac_g_ex.fe_group)
> +			atomic_inc(&sbi->s_bal_stream_goals);
> +	}
>  out:
>  	if (!err && ac->ac_status != AC_STATUS_FOUND && first_err)
>  		err = first_err;
> @@ -3194,6 +3199,8 @@ int ext4_seq_mb_stats_show(struct seq_file *seq, void *offset)
>  	seq_printf(seq, "\textents_scanned: %u\n",
>  		   atomic_read(&sbi->s_bal_ex_scanned));
>  	seq_printf(seq, "\t\tgoal_hits: %u\n", atomic_read(&sbi->s_bal_goals));
> +	seq_printf(seq, "\t\tstream_goal_hits: %u\n",
> +		   atomic_read(&sbi->s_bal_stream_goals));
>  	seq_printf(seq, "\t\tlen_goal_hits: %u\n",
>  		   atomic_read(&sbi->s_bal_len_goals));
>  	seq_printf(seq, "\t\t2^n_hits: %u\n", atomic_read(&sbi->s_bal_2orders));
> -- 
> 2.46.1
>