Re: [PATCH net-next v25 00/13] Device Memory TCP

Mina Almasry
2024-09-09T16:54:30Z

On Mon, Sep 9, 2024 at 4:21 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> On 2024/9/9 13:43, 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.
>
> Sorry for the late report, as I was adding a testing page_pool ko basing
> on [1] to avoid introducing performance regression when fixing the bug in
> [2].
> I used it to test the performance impact of devmem patchset for page_pool
> too, it seems there might be some noticable performance impact quite stably
> for the below testcases, about 5%~16% performance degradation as below in
> the arm64 system:
>

Correct me if I'm wrong here, but on the surface here it seems that
you're re-reporting a known issue. Consensus seems to be that it's a
non-issue.

In v6 I reported that the bench_page_pool_simple.ko test reports a 1
cycle regression with these patches, from 8->9 cycles. That is roughly
consistent with the 5-15% you're reporting.

I root caused the reason for the regression to be the
netmem_is_net_iov() check in the fast path. I removed this regression
in v7 (see the change log) by conditionally compiling the check in
that function.

In v8, Pavel/Jens/David pushed back on the ifdef check. See this
entire thread, but in particular this response from Jens:

https://lore.kernel.org/lkml/11f52113-7b67-4b45-ba1d-29b070050cec@kernel.dk/

Seems consensus that it's 'not really worth it in this scenario'.

--
Thanks,
Mina