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. -- Thanks, Mina
