On 2024/3/6 3:38, Mina Almasry wrote: > On Tue, Mar 5, 2024 at 4:54 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote: >> >> On 2024/3/5 10:01, 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. >>> >> >> The last sentence seems to be suggesting the above 1 ns regresses is caused >> by the static_branch_unlikely() checking? > > Note it's not a 1ns regression, it's looks like maybe a 1 cycle > regression (slightly less than 1ns if I'm reading the output of the > test correctly): > > # clean net-next > time_bench: Type:tasklet_page_pool01_fast_path Per elem: 8 cycles(tsc) > 2.993 ns (step:0) > > # with patches > time_bench: Type:tasklet_page_pool01_fast_path Per elem: 9 cycles(tsc) > 3.679 ns (step:0) > > # with patches and with diff that disables static branching: > time_bench: Type:tasklet_page_pool01_fast_path Per elem: 8 cycles(tsc) > 3.248 ns (step:0) > > I do see noise in the test results between run and run, and any > regression (if any) is slightly obfuscated by the noise, so it's a bit > hard to make confident statements. So far it looks like a ~0.25ns > regression without static branch and about ~0.65ns with static branch. > > Honestly when I saw all 3 results were within some noise I did not > investigate more, but if this looks concerning to you I can dig > further. I likely need to gather a few test runs to filter out the > noise and maybe investigate the assembly my compiler is generating to > maybe narrow down what changes there. Yes, that is confusing enough that need more investigation. >
