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
