On Tue, Mar 26, 2024 at 5:47 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote: > > On 2024/3/26 8:28, Mina Almasry wrote: > > On Tue, Mar 5, 2024 at 11:38 AM Mina Almasry <almasrymina@xxxxxxxxxx> 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. > >> > > > > I did some more investigation here to gather more data to filter out > > the noise, and recorded the summary here: > > > > https://pastebin.com/raw/v5dYRg8L > > > > Long story short, the page_pool benchmark results are consistent with > > some outlier noise results that I'm discounting here. Currently > > page_pool fast path is at 8 cycles > > > > [ 2115.724510] time_bench: Type:tasklet_page_pool01_fast_path Per > > elem: 8 cycles(tsc) 3.187 ns (step:0) - (measurement period > > time:0.031870585 sec time_interval:31870585) - (invoke count:10000000 > > tsc_interval:86043192) > > > > and with this patch series it degrades to 10 cycles, or about a 0.7ns > > degradation or so: > > Even if the absolute value for the overhead is small, we seems have a > degradation of about 20% for tasklet_page_pool01_fast_path testcase, > which seems scary. > > I am assuming that every page is recyclable for tasklet_page_pool01_fast_path > testcase, and that code path matters for page_pool, it would be good to > remove any additional checking for that code path. > We can remove the usage of static_branch_unlikely in the net_iov check, which reduces the overhead to 1 cycle (8->9), only 12.5% overhead. The addition of the static_branch_unlikely is not improving the performance of devmem TCP anyway. From previous discussions with Jesper he deemed a 1 cycle degradation acceptable, but he hasn't commented in a while, he may have changed his mind but so far no complaints. We can additionally only add the check only if CONFIG_SHARED_DMA_BUFFER is enabled. I've tested that and the fast path goes back to 8 cycles (0 overhead). If CONFIG_SHARED_DMA_BUFFER is not enabled then netmem can't be dmabuf anyway, so no reason to check. > And we already have pool->has_init_callback checking when we have to use > a new page, it may make sense to refactor that to share the same checking > for provider to avoid the overhead as much as possible. > > Also, I am not sure if it really matter that much, as with the introducing > of netmem_is_net_iov() checking spreading in the networking, the overhead > might add up for other case too. -- Thanks, Mina
