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Message-ID: <6f281517-3785-ce46-65de-e2f78576783b@intel.com>
Date: Fri, 18 Oct 2019 17:45:26 -0700
From: "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Toke Høiland-Jørgensen <toke@...hat.com>
Cc: 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 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
In the archives, i see that Toke had some comments, but somehow i didn't get his email in
my inbox.
> Performance Results
> ===================
> Only 1 core is used in all these testcases as the app and the queue irq are pinned to the same core.
>
> ----------------------------------------------------------------------------------
> mitigations ON mitigations OFF
> Testcase ----------------------------------------------------------
> no patches with patches no patches with patches
> ----------------------------------------------------------------------------------
> AF_XDP default rxdrop X X Y Y
> Is this really exactly the same with and without patches? You're adding
> an extra check to xdp_do_redirect(); are you really saying that the
> impact of that is zero?
Yes. I didn't see any impact. The variation is within +/- < 1%
I could use static_key even for that check in xdp_do_redirect() if required.
-Sridhar
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