Date: Mon, 25 Mar 2024 17:28:12 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, YiFei Zhu <zhuyifei@...gle.com>
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Subject: Re: [RFC PATCH net-next v6 00/15] Device Memory TCP

On Tue, Mar 5, 2024 at 11:38 AM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Tue, Mar 5, 2024 at 4:54 AM Yunsheng Lin <linyunsheng@...wei.com> 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:

[  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.

-- 
Thanks,
Mina

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