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Date: Fri, 18 Oct 2019 19:25:27 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
Jakub Kicinski <jakub.kicinski@...ronome.com>,
"Karlsson, Magnus" <magnus.karlsson@...el.com>,
Björn Töpel <bjorn.topel@...el.com>,
Netdev <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
"Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
"Herbert, Tom" <tom.herbert@...el.com>
Subject: Re: FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive
packets directly from a queue
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.
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