Re: [Regression] ext4: changes to mb_optimize_scan cause issues on Raspberry Pi

[Stefan Wahren]

Hi Ojaswin,

Am 26.07.22 um 08:43 schrieb Ojaswin Mujoo:
> On Mon, Jul 25, 2022 at 09:09:32PM +0200, Stefan Wahren wrote:
> > Hi Ojaswin,
> >
> > Am 25.07.22 um 17:07 schrieb Ojaswin Mujoo:
> > > 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@xxxxxxxxxxxxx>
> > > > 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.
> > thanks. I wasn't able to reproduce this heavy load symptoms with every SD
> > card. Maybe this depends on the write performance of the SD card to trigger
> > the situation (used command to measure write performance: dd if=/dev/zero
> > of=/boot/test bs=1M count=30 oflag=dsync,direct ).
> >
> > I tested a Kingston consumer 32 GB which had nearly constant write
> > performance of 13 MB/s and didn't had the heavy load symptoms. The firmware
> > update was done in a few seconds, so hard to say that at least the
> > performance regression is reproducible.
> >
> > I also tested 2x Kingston industrial 16 GB which had a floating write
> > performance between 5 and 10 MB/s (wear leveling?) and both had the heavy
> > load symptoms.
> >
> > All SD cards has been detected as ultra high speed DDR50 by the emmc2
> > interface.
> >
> > Best regards
> >
> > > Regard,
> > > Ojaswin
> > >
> > > [1]
> > > 	commit 196e402adf2e4cd66f101923409f1970ec5f1af3
> > > 	From: Harshad Shirwadkar <harshadshirwadkar@xxxxxxxxx>
> > > 	Date: Thu, 1 Apr 2021 10:21:27 -0700
> > > 	
> > > 	ext4: improve cr 0 / cr 1 group scanning
> > >
> > > > Regards
> Thanks for the info Stefan, I'm still trying to reproduce the issue but
> it's slightly challenging since I don't have my RPi handy at the moment.
>
> In the meantime, would you please try out the mb_optmize_scan=0 command
> line options to see if that helps bypass the issue. This will help
> confirm if the issue lies in mb_optmize_scan itself or if its something
> else.
I run the firmware update 5 times with mb_optimize_scan=0 on my 
Raspberry Pi 4 and the industrial SD card and everytime the update worked.
> Regards,
> Ojaswin