Re: [PATCH v2 01/12] Revert "ext4: remove ac->ac_found > sbi->s_mb_min_to_scan dead check in ext4_mb_check_limits"

[Thorsten Leemhuis]

On 31.05.23 10:57, Ojaswin Mujoo wrote:
> On Tue, May 30, 2023 at 06:28:22PM +0200, Sedat Dilek wrote:
>> On Tue, May 30, 2023 at 3:25 PM Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> wrote:
>>>
>>> This reverts commit 32c0869370194ae5ac9f9f501953ef693040f6a1.
>>>
>>> The reverted commit was intended to remove a dead check however it was observed
>>> that this check was actually being used to exit early instead of looping
>>> sbi->s_mb_max_to_scan times when we are able to find a free extent bigger than
>>> the goal extent. Due to this, a my performance tests (fsmark, parallel file
>>> writes in a highly fragmented FS) were seeing a 2x-3x regression.
>>>
>>> Example, the default value of the following variables is:
>>>
>>> sbi->s_mb_max_to_scan = 200
>>> sbi->s_mb_min_to_scan = 10
>>>
>>> In ext4_mb_check_limits() if we find an extent smaller than goal, then we return
>>> early and try again. This loop will go on until we have processed
>>> sbi->s_mb_max_to_scan(=200) number of free extents at which point we exit and
>>> just use whatever we have even if it is smaller than goal extent.
>>>
>>> Now, the regression comes when we find an extent bigger than goal. Earlier, in
>>> this case we would loop only sbi->s_mb_min_to_scan(=10) times and then just use
>>> the bigger extent. However with commit 32c08693 that check was removed and hence
>>> we would loop sbi->s_mb_max_to_scan(=200) times even though we have a big enough
>>> free extent to satisfy the request. The only time we would exit early would be
>>> when the free extent is *exactly* the size of our goal, which is pretty uncommon
>>> occurrence and so we would almost always end up looping 200 times.
>>>
>>> Hence, revert the commit by adding the check back to fix the regression. Also
>>> add a comment to outline this policy.
>>
>> I applied this single patch of your series v2 on top of Linux v6.4-rc4.
>>
>> So, if this is a regression I ask myself if this is material for Linux 6.4?
>>
>> Can you comment on this, please?
> 
> Since this patch fixes a regression I think it should ideally go in
> Linux 6.4

Ted can speak up for himself, but maybe this might speed things up:

A lot of maintainers in a case like this want fixes (like this)
submitted separately from other changes (like the rest of this series).

/me hopes this will help and not confuse anything

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
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