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Message-ID: <CAJ+HfNj07FwmU2GGpUYw56PRwu4pHyHNSkbCOogbMB5zB2QqWA@mail.gmail.com>
Date: Sun, 20 Oct 2019 19:12:01 +0200
From: Björn Töpel <bjorn.topel@...il.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
"Samudrala, Sridhar" <sridhar.samudrala@...el.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 Sun, 20 Oct 2019 at 12:15, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Fri, Oct 18, 2019 at 05:45:26PM -0700, Samudrala, Sridhar wrote:
> >> On 10/18/2019 5:14 PM, Alexei Starovoitov wrote:
> >> > On Fri, Oct 18, 2019 at 11:40:07AM -0700, Samudrala, Sridhar wrote:
> >> > >
> >> > > Perf report for "AF_XDP default rxdrop" with patched kernel - mitigations ON
> >> > > ==========================================================================
> >> > > Samples: 44K of event 'cycles', Event count (approx.): 38532389541
> >> > > Overhead Command Shared Object Symbol
> >> > > 15.31% ksoftirqd/28 [i40e] [k] i40e_clean_rx_irq_zc
> >> > > 10.50% ksoftirqd/28 bpf_prog_80b55d8a76303785 [k] bpf_prog_80b55d8a76303785
> >> > > 9.48% xdpsock [i40e] [k] i40e_clean_rx_irq_zc
> >> > > 8.62% xdpsock xdpsock [.] main
> >> > > 7.11% ksoftirqd/28 [kernel.vmlinux] [k] xsk_rcv
> >> > > 5.81% ksoftirqd/28 [kernel.vmlinux] [k] xdp_do_redirect
> >> > > 4.46% xdpsock bpf_prog_80b55d8a76303785 [k] bpf_prog_80b55d8a76303785
> >> > > 3.83% xdpsock [kernel.vmlinux] [k] xsk_rcv
> >> >
> >> > why everything is duplicated?
> >> > Same code runs in different tasks ?
> >>
> >> Yes. looks like these functions run from both the app(xdpsock) context and ksoftirqd context.
> >>
> >> >
> >> > > 2.81% ksoftirqd/28 [kernel.vmlinux] [k] bpf_xdp_redirect_map
> >> > > 2.78% ksoftirqd/28 [kernel.vmlinux] [k] xsk_map_lookup_elem
> >> > > 2.44% xdpsock [kernel.vmlinux] [k] xdp_do_redirect
> >> > > 2.19% ksoftirqd/28 [kernel.vmlinux] [k] __xsk_map_redirect
> >> > > 1.62% ksoftirqd/28 [kernel.vmlinux] [k] xsk_umem_peek_addr
> >> > > 1.57% xdpsock [kernel.vmlinux] [k] xsk_umem_peek_addr
> >> > > 1.32% ksoftirqd/28 [kernel.vmlinux] [k] dma_direct_sync_single_for_cpu
> >> > > 1.28% xdpsock [kernel.vmlinux] [k] bpf_xdp_redirect_map
> >> > > 1.15% xdpsock [kernel.vmlinux] [k] dma_direct_sync_single_for_device
> >> > > 1.12% xdpsock [kernel.vmlinux] [k] xsk_map_lookup_elem
> >> > > 1.06% xdpsock [kernel.vmlinux] [k] __xsk_map_redirect
> >> > > 0.94% ksoftirqd/28 [kernel.vmlinux] [k] dma_direct_sync_single_for_device
> >> > > 0.75% ksoftirqd/28 [kernel.vmlinux] [k] __x86_indirect_thunk_rax
> >> > > 0.66% ksoftirqd/28 [i40e] [k] i40e_clean_programming_status
> >> > > 0.64% ksoftirqd/28 [kernel.vmlinux] [k] net_rx_action
> >> > > 0.64% swapper [kernel.vmlinux] [k] intel_idle
> >> > > 0.62% ksoftirqd/28 [i40e] [k] i40e_napi_poll
> >> > > 0.57% xdpsock [kernel.vmlinux] [k] dma_direct_sync_single_for_cpu
> >> > >
> >> > > Perf report for "AF_XDP direct rxdrop" with patched kernel - mitigations ON
> >> > > ==========================================================================
> >> > > Samples: 46K of event 'cycles', Event count (approx.): 38387018585
> >> > > Overhead Command Shared Object Symbol
> >> > > 21.94% ksoftirqd/28 [i40e] [k] i40e_clean_rx_irq_zc
> >> > > 14.36% xdpsock xdpsock [.] main
> >> > > 11.53% ksoftirqd/28 [kernel.vmlinux] [k] xsk_rcv
> >> > > 11.32% xdpsock [i40e] [k] i40e_clean_rx_irq_zc
> >> > > 4.02% xdpsock [kernel.vmlinux] [k] xsk_rcv
> >> > > 2.91% ksoftirqd/28 [kernel.vmlinux] [k] xdp_do_redirect
> >> > > 2.45% ksoftirqd/28 [kernel.vmlinux] [k] xsk_umem_peek_addr
> >> > > 2.19% xdpsock [kernel.vmlinux] [k] xsk_umem_peek_addr
> >> > > 2.08% ksoftirqd/28 [kernel.vmlinux] [k] bpf_direct_xsk
> >> > > 2.07% ksoftirqd/28 [kernel.vmlinux] [k] dma_direct_sync_single_for_cpu
> >> > > 1.53% ksoftirqd/28 [kernel.vmlinux] [k] dma_direct_sync_single_for_device
> >> > > 1.39% xdpsock [kernel.vmlinux] [k] dma_direct_sync_single_for_device
> >> > > 1.22% ksoftirqd/28 [kernel.vmlinux] [k] xdp_get_xsk_from_qid
> >> > > 1.12% ksoftirqd/28 [i40e] [k] i40e_clean_programming_status
> >> > > 0.96% ksoftirqd/28 [i40e] [k] i40e_napi_poll
> >> > > 0.95% ksoftirqd/28 [kernel.vmlinux] [k] net_rx_action
> >> > > 0.89% xdpsock [kernel.vmlinux] [k] xdp_do_redirect
> >> > > 0.83% swapper [i40e] [k] i40e_clean_rx_irq_zc
> >> > > 0.70% swapper [kernel.vmlinux] [k] intel_idle
> >> > > 0.66% xdpsock [kernel.vmlinux] [k] dma_direct_sync_single_for_cpu
> >> > > 0.60% xdpsock [kernel.vmlinux] [k] bpf_direct_xsk
> >> > > 0.50% ksoftirqd/28 [kernel.vmlinux] [k] xsk_umem_discard_addr
> >> > >
> >> > > Based on the perf reports comparing AF_XDP default and direct rxdrop, we can say that
> >> > > AF_XDP direct rxdrop codepath is avoiding the overhead of going through these functions
> >> > > bpf_prog_xxx
> >> > > bpf_xdp_redirect_map
> >> > > xsk_map_lookup_elem
> >> > > __xsk_map_redirect
> >> > > With AF_XDP direct, xsk_rcv() is directly called via bpf_direct_xsk() in xdp_do_redirect()
> >> >
> >> > I don't think you're identifying the overhead correctly.
> >> > xsk_map_lookup_elem is 1%
> >> > but bpf_xdp_redirect_map() suppose to call __xsk_map_lookup_elem()
> >> > which is a different function:
> >> > ffffffff81493fe0 T __xsk_map_lookup_elem
> >> > ffffffff81492e80 t xsk_map_lookup_elem
> >> >
> >> > 10% for bpf_prog_80b55d8a76303785 is huge.
> >> > It's the actual code of the program _without_ any helpers.
> >> > How does the program actually look?
> >>
> >> It is the xdp program that is loaded via xsk_load_xdp_prog() in tools/lib/bpf/xsk.c
> >> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/lib/bpf/xsk.c#n268
> >
> > I see. Looks like map_gen_lookup was never implemented for xskmap.
> > How about adding it first the way array_map_gen_lookup() is implemented?
> > This will easily give 2x perf gain.
>
> I guess we should implement this for devmaps as well now that we allow
> lookups into those.
>
> However, in this particular example, the lookup from BPF is not actually
> needed, since bpf_redirect_map() will return a configurable error value
> when the map lookup fails (for exactly this use case).
>
> So replacing:
>
> if (bpf_map_lookup_elem(&xsks_map, &index))
> return bpf_redirect_map(&xsks_map, index, 0);
>
> with simply
>
> return bpf_redirect_map(&xsks_map, index, XDP_PASS);
>
> would save the call to xsk_map_lookup_elem().
>
Thanks for the reminder! I just submitted a patch. Still, doing the
map_gen_lookup() for xsk/devmaps still makes sense!
Björn
> -Toke
>
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