netdev - Re: [Intel-wired-lan] FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive packets directly from a queue

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Message-ID: <CAADnVQKwnMChzeGaC66A99cHn5szB4hPZaGXq8JAhd8sjrdGeA@mail.gmail.com>
Date:   Wed, 23 Oct 2019 10:42:00 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Netdev <netdev@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        "Herbert, Tom" <tom.herbert@...el.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [Intel-wired-lan] FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP
 sockets to receive packets directly from a queue

On Tue, Oct 22, 2019 at 12:06 PM Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
>
> OK. Here is another data point that shows the perf report with the same test but CPU mitigations
> turned OFF. Here bpf_prog overhead goes down from almost (10.18 + 4.51)% to (3.23 + 1.44%).
>
>    21.40%  ksoftirqd/28     [i40e]                     [k] i40e_clean_rx_irq_zc
>    14.13%  xdpsock          [i40e]                     [k] i40e_clean_rx_irq_zc
>     8.33%  ksoftirqd/28     [kernel.vmlinux]           [k] xsk_rcv
>     6.09%  ksoftirqd/28     [kernel.vmlinux]           [k] xdp_do_redirect
>     5.19%  xdpsock          xdpsock                    [.] main
>     3.48%  ksoftirqd/28     [kernel.vmlinux]           [k] bpf_xdp_redirect_map
>     3.23%  ksoftirqd/28     bpf_prog_3c8251c7e0fef8db  [k] bpf_prog_3c8251c7e0fef8db
>
> So a major component of the bpf_prog overhead seems to be due to the CPU vulnerability mitigations.

I feel that it's an incorrect conclusion because JIT is not doing
any retpolines (because there are no indirect calls in bpf).
There should be no difference in bpf_prog runtime with or without mitigations.
Also you're running root, so no spectre mitigations either.

This 3% seems like a lot for a function that does few loads that should
hit d-cache and one direct call.
Please investigate why you're seeing this 10% cpu cost when mitigations are on.
perf report/annotate is the best.
Also please double check that you're using the latest perf.
Since bpf performance analysis was greatly improved several versions ago.
I don't think old perf will be showing bogus numbers, but better to
run the latest.

> The other component is the bpf_xdp_redirect_map() codepath.
>
> Let me know if it helps to collect any other data that should further help with the perf analysis.
>