On Mon, Sep 9, 2024 at 4:21 AM Yunsheng Lin linyunsheng@huawei.com wrote:
On 2024/9/9 13:43, Mina Almasry wrote:
Perf - page-pool benchmark:
bench_page_pool_simple.ko tests with and without these changes: https://pastebin.com/raw/ncHDwAbn
AFAIK the number that really matters in the perf tests is the 'tasklet_page_pool01_fast_path Per elem'. This one measures at about 8 cycles without the changes but there is some 1 cycle noise in some results.
With the patches this regresses to 9 cycles with the changes but there is 1 cycle noise occasionally running this test repeatedly.
Lastly I tried disable the static_branch_unlikely() in netmem_is_net_iov() check. To my surprise disabling the static_branch_unlikely() check reduces the fast path back to 8 cycles, but the 1 cycle noise remains.
Sorry for the late report, as I was adding a testing page_pool ko basing on [1] to avoid introducing performance regression when fixing the bug in [2]. I used it to test the performance impact of devmem patchset for page_pool too, it seems there might be some noticable performance impact quite stably for the below testcases, about 5%~16% performance degradation as below in the arm64 system:
Correct me if I'm wrong here, but on the surface here it seems that you're re-reporting a known issue. Consensus seems to be that it's a non-issue.
In v6 I reported that the bench_page_pool_simple.ko test reports a 1 cycle regression with these patches, from 8->9 cycles. That is roughly consistent with the 5-15% you're reporting.
I root caused the reason for the regression to be the netmem_is_net_iov() check in the fast path. I removed this regression in v7 (see the change log) by conditionally compiling the check in that function.
In v8, Pavel/Jens/David pushed back on the ifdef check. See this entire thread, but in particular this response from Jens:
https://lore.kernel.org/lkml/11f52113-7b67-4b45-ba1d-29b070050cec@kernel.dk/
Seems consensus that it's 'not really worth it in this scenario'.