Re: [RFC] Significant Random I/O Performance Regression in Linux Kernel 6.18 (Up to 27.7%) Likely Caused by Commit 3c7ac40d7322

[André Draszik]

Hi,

On Fri, 2026-07-10 at 07:17 +0000, 孙魁 (Kui Sun) wrote:
> Dear Kernel Maintainers,
> 
> During our upgrade from Linux kernel 5.15 to Linux kernel 6.18, we observed a significant performance regression in random I/O
> workloads—with a maximum degradation of 27.7%.

[...]

> This issue is particularly pronounced in single-threaded, small-block I/O scenarios。
> 
> To illustrate the impact, we conducted benchmark tests using AnTuTu on Unisoc T615 devices.
> The results are summarized below:

[...]


> Root Cause Identification
> 
> Through investigation, we identified that upstream commit 3c7ac40d732232fec0ba31d0a5e3cc9c112fc2e7, merged in April 2025, is likely
> responsible for this performance drop.
> After locally reverting this commit on kernel 6.18, performance fully recovered:
> 
> Table 4：Mixed Random Read/Write Speed Scores（After Revert）
> Device  Kernel Version                  Test1   Test2   Test3   Average
> T615    5.15                                    18604   18314   17732   18216.67
> T615    6.18                            13372   13081   13081   13178.00（↓27.66%）
> T615    6.18（reverted 3c7ac40)  18314   18604   18604   18507.33
> 

Thank you for your above analysis. Your numbers match up well with my own
observations at the time in
https://lore.kernel.org/all/88d31a258feb36425ad73d0323077972f85f8341.camel@xxxxxxxxxx/

[...]

> Request and Recommendations
> 
> Given the tangible impact on mobile user experience, we kindly request the community to:
> 1.      Consider reverting commit 3c7ac40d732232fec0ba31d0a5e3cc9c112fc2e7, or
> 2.      Re-evaluate the proposed change in light of its effect on low-concurrency I/O paths, as discussed here:
> https://lore.kernel.org/lkml/88d31a258feb36425ad73d0323077972f85f8341.camel@xxxxxxxxxx/

While originally I was trying to find a solution that doesn't regress existing
platforms and still works for the newly added platform, I can only second the request
for revert, given how important UFS is for mobile.

Mani was also in favour of reverting:
https://lore.kernel.org/all/4enen7mopxtx4ijl5qyrd2gnxvv3kygtlnhxpr64egckpvkja4@hjli25ndhxwc/


Cheers,
Andre'