linux-ext4 - [Bug 217965] ext4(?) regression since 6.5.0 on sata hdd

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Message-ID: <bug-217965-13602-oNiK3hL8DX@https.bugzilla.kernel.org/>
Date: Thu, 04 Jan 2024 05:38:36 +0000
From: bugzilla-daemon@...nel.org
To: linux-ext4@...r.kernel.org
Subject: [Bug 217965] ext4(?) regression since 6.5.0 on sata hdd

https://bugzilla.kernel.org/show_bug.cgi?id=217965

--- Comment #61 from Ojaswin Mujoo (ojaswin.mujoo@....com) ---
Hi Matthew, thanks for confirming. So as pointed out in comment 9 [1], for the
above steps to disable CR_BEST_AVAIL_LEN code did not fix the issue. My
suspicion is that this issue can occur either in CR_GOAL_LEN_FAST or
CR_BEST_AVAIL_LEN lookup, depending on the block groups being searched.
Probably for you, it was occurring during CR_BEST_AVAIL_LEN lookup and hence
disabling that code fixed it. 

Further, as Carlos pointed out above, they are able to see this in all 6.*
kernels which means this is happening before CR_BEST_AVAIL_LEN was introduced
however it seems to be much easier to trigger in 6.5+ kernels. Now, assuming
the above theory is correct, then in cases where this is triggered from
CR_GOAL_LEN_FAST, it should ideally happen easily in pre 6.5 kernels as well,
but it doesn't, which makes me think that there might be some other related
changes in 6.5 that might be making it easier to trigger.

I'll try to play around a bit more with this. Also, as for higher CPU usage,
how high are we talking about? So CR_BEST_AVAIL_LEN does add some extra cycles
at the cost of generally faster allocation in fragmented filesystems, however
since you have disabled it we shouldn't ideally be seeing it. Also, does the
CPU util consistently drop when you commented out that code?

[1] https://bugzilla.kernel.org/show_bug.cgi?id=217965#c9

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