linux-kernel - Re: [Regression] ext4: changes to mb_optimize

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Date:   Mon, 25 Jul 2022 20:37:31 +0530
From:   Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     linux-ext4@...r.kernel.org,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Geetika.Moolchandani1@....com, regressions@...ts.linux.dev
Subject: Re: [Regression] ext4: changes to mb_optimize_scan cause issues on
 Raspberry Pi

On Mon, Jul 18, 2022 at 03:29:47PM +0200, Stefan Wahren wrote:
> Hi,
> 
> i noticed that since Linux 5.18 (Linux 5.19-rc6 is still affected) i'm
> unable to run "rpi-update" without massive performance regression on my
> Raspberry Pi 4 (multi_v7_defconfig + CONFIG_ARM_LPAE). Using Linux 5.17 this
> tool successfully downloads the latest firmware (> 100 MB) on my development
> micro SD card (Kingston 16 GB Industrial) with a ext4 filesystem within ~ 1
> min. The same scenario on Linux 5.18 shows the following symptoms:
> 
> - download takes endlessly much time and leads to an abort by userspace in
> most cases because of the poor performance
> - massive system load during download even after download has been aborted
> (heartbeat LED goes wild)
> - whole system becomes nearly unresponsive
> - system load goes back to normal after > 10 min
> - dmesg doesn't show anything suspicious
> 
> I was able to bisect this issue:
> 
> ff042f4a9b050895a42cae893cc01fa2ca81b95c good
> 4b0986a3613c92f4ec1bdc7f60ec66fea135991f bad
> 25fd2d41b505d0640bdfe67aa77c549de2d3c18a bad
> b4bc93bd76d4da32600795cd323c971f00a2e788 bad
> 3fe2f7446f1e029b220f7f650df6d138f91651f2 bad
> b080cee72ef355669cbc52ff55dc513d37433600 good
> ad9c6ee642a61adae93dfa35582b5af16dc5173a good
> 9b03992f0c88baef524842e411fbdc147780dd5d bad
> aab4ed5816acc0af8cce2680880419cd64982b1d good
> 14705fda8f6273501930dfe1d679ad4bec209f52 good
> 5c93e8ecd5bd3bfdee013b6da0850357eb6ca4d8 good
> 8cb5a30372ef5cf2b1d258fce1711d80f834740a bad
> 077d0c2c78df6f7260cdd015a991327efa44d8ad bad
> cc5095747edfb054ca2068d01af20be3fcc3634f good
> 27b38686a3bb601db48901dbc4e2fc5d77ffa2c1 good
> 
> commit 077d0c2c78df6f7260cdd015a991327efa44d8ad
> Author: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> Date:   Tue Mar 8 15:22:01 2022 +0530
> 
> ext4: make mb_optimize_scan performance mount option work with extents
> 
> If i revert this commit with Linux 5.19-rc6 the performance regression
> disappears.
> 
> Please ask if you need more information.

Hi Stefan, 

Apologies, I had missed this email initially. So this particular patch
simply changed a typo in an if condition which was preventing the
mb_optimize_scan option to be enabled correctly (This feature was
introduced in the following commit [1]). I think with the
mb_optimize_scan now working, it is somehow causing the firmware
download/update to take a longer time. 

I'll try to investigate this and get back with my findings.

Regard,
Ojaswin

[1] 
	commit 196e402adf2e4cd66f101923409f1970ec5f1af3
	From: Harshad Shirwadkar <harshadshirwadkar@...il.com>
	Date: Thu, 1 Apr 2021 10:21:27 -0700
	
	ext4: improve cr 0 / cr 1 group scanning

> 
> Regards
>