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

[Stefan Wahren <stefan.wahren@i2se.com>]

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@linux.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.
>> 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@gmail.com>
>>> 	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