Re: FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive packets directly from a queue

From: "Samudrala, Sridhar" <sridhar.samudrala@intel.com>
To: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: "Jakub Kicinski" <jakub.kicinski@netronome.com>,
	"Karlsson, Magnus" <magnus.karlsson@intel.com>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	Netdev <netdev@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	intel-wired-lan <intel-wired-lan@lists.osuosl.org>,
	"Fijalkowski, Maciej" <maciej.fijalkowski@intel.com>,
	"Herbert, Tom" <tom.herbert@intel.com>
Subject: Re: FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive packets directly from a queue
Date: Fri, 18 Oct 2019 17:45:26 -0700	[thread overview]
Message-ID: <6f281517-3785-ce46-65de-e2f78576783b@intel.com> (raw)
In-Reply-To: <20191019001449.fk3gnhih4nx724pm@ast-mbp>

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