netdev - Re: [PATCH net-next v2 0/3] net/sched: retpoline wrappers for tc

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Date:   Thu, 1 Dec 2022 07:34:39 -0500
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Pedro Tammela <pctammela@...il.com>, netdev@...r.kernel.org,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        xiyou.wangcong@...il.com, jiri@...nulli.us, kuniyu@...zon.com,
        Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH net-next v2 0/3] net/sched: retpoline wrappers for tc

On Thu, Dec 1, 2022 at 6:05 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Mon, 2022-11-28 at 12:44 -0300, Pedro Tammela wrote:

[..]

> > We observed a 3-6% speed up on the retpoline CPUs, when going through 1
> > tc filter,
>
> Do yu have all the existing filters enabled at build time in your test
> kernel? the reported figures are quite higher then expected considering
> there are 7th new unlikely branch in between.
>

That can be validated with a test that compiles a kernel with a filter under
test listed first then another kernel with the same filter last.

Also given these tests were using 64B pkts to achieve the highest pps, perhaps
using MTU sized pkts with pktgen would give more realistic results?

In addition to the tests for 1 and 100 filters...

> Also it would be nice to have some figure for the last filter in the if
> chain. I fear we could have some regressions there even for 'retpoline'
> CPUs - given the long if chain - and u32 is AFAIK (not much actually)
> still quite used.
>

I would say flower and bpf + u32 are probably the highest used,
but given no available data on this usage beauty is in the eye of
the beholder. I hope it doesnt become a real estate battle like we
have in which subsystem gets to see packets first or last ;->

> Finally, it looks like the filter order in patch 1/3 is quite relevant,
> and it looks like you used the lexicographic order, I guess it should
> be better to sort them by 'relevance', if someone could provide a
> reasonable 'relevance' order. I personally would move ife, ipt and
> simple towards the bottom.

I think we can come up with some reasonable order.

cheers,
jamal