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Message-ID: <CAADnVQ+jiEO+jnFR-G=xG=zz7UOSBieZbc1NN=sSnAwvPaJjUQ@mail.gmail.com>
Date: Mon, 21 Oct 2019 15:34:18 -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 Mon, Oct 21, 2019 at 1:10 PM Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
>
> On 10/20/2019 10:12 AM, Björn Töpel wrote:
> > 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!
> >
>
> I tried Bjorn's patch that avoids the lookups in the BPF prog.
> https://lore.kernel.org/netdev/20191021105938.11820-1-bjorn.topel@gmail.com/
>
> With this patch I am also seeing around 3-4% increase in xdpsock rxdrop performance and
> the perf report looks like this.
>
> Samples: 44K of event 'cycles', Event count (approx.): 38749965204
> Overhead Command Shared Object Symbol
> 16.06% ksoftirqd/28 [i40e] [k] i40e_clean_rx_irq_zc
> 10.18% ksoftirqd/28 bpf_prog_3c8251c7e0fef8db [k] bpf_prog_3c8251c7e0fef8db
> 10.15% xdpsock [i40e] [k] i40e_clean_rx_irq_zc
> 10.06% ksoftirqd/28 [kernel.vmlinux] [k] xsk_rcv
> 7.45% xdpsock xdpsock [.] main
> 5.76% ksoftirqd/28 [kernel.vmlinux] [k] xdp_do_redirect
> 4.51% xdpsock bpf_prog_3c8251c7e0fef8db [k] bpf_prog_3c8251c7e0fef8db
> 3.67% xdpsock [kernel.vmlinux] [k] xsk_rcv
> 3.06% ksoftirqd/28 [kernel.vmlinux] [k] bpf_xdp_redirect_map
> 2.34% ksoftirqd/28 [kernel.vmlinux] [k] __xsk_map_redirect
> 2.33% xdpsock [kernel.vmlinux] [k] xdp_do_redirect
> 1.69% ksoftirqd/28 [kernel.vmlinux] [k] xsk_umem_peek_addr
> 1.69% xdpsock [kernel.vmlinux] [k] xsk_umem_peek_addr
> 1.42% ksoftirqd/28 [kernel.vmlinux] [k] dma_direct_sync_single_for_cpu
> 1.19% xdpsock [kernel.vmlinux] [k] bpf_xdp_redirect_map
> 1.13% xdpsock [kernel.vmlinux] [k] dma_direct_sync_single_for_device
> 0.95% ksoftirqd/28 [kernel.vmlinux] [k] dma_direct_sync_single_for_device
> 0.92% swapper [kernel.vmlinux] [k] intel_idle
> 0.92% xdpsock [kernel.vmlinux] [k] __xsk_map_redirect
> 0.80% ksoftirqd/28 [kernel.vmlinux] [k] __x86_indirect_thunk_rax
> 0.73% ksoftirqd/28 [i40e] [k] i40e_clean_programming_status
> 0.71% ksoftirqd/28 [kernel.vmlinux] [k] __xsk_map_lookup_elem
> 0.63% ksoftirqd/28 [kernel.vmlinux] [k] net_rx_action
> 0.62% ksoftirqd/28 [i40e] [k] i40e_napi_poll
> 0.58% xdpsock [kernel.vmlinux] [k] dma_direct_sync_single_for_cpu
>
> So with this patch applied, direct receive performance improvement comes down from 46% to 42%.
> I think it is still substantial enough to provide an option to allow direct receive for
> certain use cases. If it is OK, i can re-spin and submit the patches on top of the latest bpf-next
I think it's too early to consider such drastic approach.
The run-time performance of XDP program should be the same as C code.
Something fishy in these numbers, since spending 10% cpu in few loads
and single call to bpf_xdp_redirect_map() just not right.
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