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Message-ID: <7642a460-9ba3-d9f7-6cf8-aac45c7eef0d@intel.com>
Date: Mon, 21 Oct 2019 13:10:26 -0700
From: "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To: Björn Töpel <bjorn.topel@...il.com>,
Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.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 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
Thanks
Sridhar
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