Re: [PATCH v3 02/17] ext4: separate stream goal hits from s_bal_goals for better tracking

[Baokun Li <libaokun1@huawei.com>]

On 2025/7/17 18:29, Ojaswin Mujoo wrote:
> 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@huawei.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?

It's true that successfully acquiring the goal on retry is possible, but
the probability is too low; I think attempting the inode goal on retry is
not very meaningful. The lack of trylock logic in ext4_mb_find_by_goal()
also introduces some performance overhead.

Additionally, since preallocations might be dropped before retrying, the
inode's preallocation could also be discarded. Therefore, pa overlap needs
to be re-adjusted.

For data block allocation, we should call ext4_mb_normalize_request() to
regenerate a new ac_g_ex instead of directly resetting the original goal.
ext4_mb_normalize_request() will also determine whether to reset
EXT4_MB_HINT_TRY_GOAL.

For metadata block allocation, EXT4_MB_STREAM_ALLOC is not set, so there's
no need to worry about EXT4_MB_HINT_TRY_GOAL being cleared.

Clearing EXT4_MB_HINT_TRY_GOAL here is only to avoid inode goal allocation
with -1. If you insist that we should attempt the inode goal on retry,
I will send a separate patch to address that.


Cheers,
Baokun