[CCing the regression list, as it should be in the loop for regressions: https://docs.kernel.org/admin-guide/reporting-regressions.html] [TLDR: I'm adding this report to the list of tracked Linux kernel regressions; the text you find below is based on a few templates paragraphs you might have encountered already in similar form. See link in footer if these mails annoy you.] On 25.05.23 13:32, Ojaswin Mujoo 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. > > Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx> > [...] Thanks for the report. To be sure the issue doesn't fall through the cracks unnoticed, I'm adding it to regzbot, the Linux kernel regression tracking bot: #regzbot ^introduced 32c0869370194ae5ac #regzbot title ext4: 2x-3x regression in performance tests #regzbot monitor: https://lore.kernel.org/all/ddcae9658e46880dfec2fb0aa61d01fb3353d202.1685449706.git.ojaswin@xxxxxxxxxxxxx/ #regzbot fix: Revert "ext4: remove ac->ac_found > sbi->s_mb_min_to_scan dead check in ext4_mb_check_limits" #regzbot ignore-activity This isn't a regression? This issue or a fix for it are already discussed somewhere else? It was fixed already? You want to clarify when the regression started to happen? Or point out I got the title or something else totally wrong? Then just reply and tell me -- ideally while also telling regzbot about it, as explained by the page listed in the footer of this mail. Developers: When fixing the issue, remember to add 'Link:' tags pointing to the report (the parent of this mail). See page linked in footer for details. Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat) -- Everything you wanna know about Linux kernel regression tracking: https://linux-regtracking.leemhuis.info/about/#tldr That page also explains what to do if mails like this annoy you.
