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. 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. > > [ 498.226127] time_bench: Type:tasklet_page_pool01_fast_path Per > elem: 10 cycles(tsc) 3.944 ns (step:0) - (measurement period > time:0.039442539 sec time_interval:39442539) - (invoke count:10000000 > tsc_interval:106485268) > > I took the time to dig into where the degradation comes from, and to > my surprise we can shave off 1 cycle in perf by removing the > static_branch_unlikely check in netmem_is_net_iov() like so: > > diff --git a/include/net/netmem.h b/include/net/netmem.h > index fe354d11a421..2b4310ac1115 100644 > --- a/include/net/netmem.h > +++ b/include/net/netmem.h > @@ -122,8 +122,7 @@ typedef unsigned long __bitwise netmem_ref; > static inline bool netmem_is_net_iov(const netmem_ref netmem) > { > #ifdef CONFIG_PAGE_POOL > - return static_branch_unlikely(&page_pool_mem_providers) && > - (__force unsigned long)netmem & NET_IOV; > + return (__force unsigned long)netmem & NET_IOV; > #else > return false; > #endif > > With this change, the fast path is 9 cycles, only a 1 cycle (~0.35ns) > regression: > > [ 199.184429] time_bench: Type:tasklet_page_pool01_fast_path Per > elem: 9 cycles(tsc) 3.552 ns (step:0) - (measurement period > time:0.035524013 sec time_interval:35524013) - (invoke count:10000000 > tsc_interval:95907775) > > I did some digging with YiFei on why the static_branch_unlikely > appears to be causing a 1 cycle regression, but could not get an > answer that makes sense. The # of instructions in > page_pool_return_page() with the static_branch_unlikely and without is > about the same in the compiled .o file, and my understanding is that > static_branch will cause code re-writing anyway so looking at the > compiled code may not be representative. > > Worthy of note is that I get ~95% line rate of devmem TCP regardless > of the static_branch_unlikely() or not, so impact of the static_branch > is not large enough to be measurable end-to-end. I'm thinking I want > to drop the static_branch_unlikely() in the next RFC since it doesn't > improve the end-to-end throughput number and is resulting in a > measurable improvement in the page pool benchmark. >
