f40bfd1679
This is a preparatory change. A follow-up patch "bpf: verify callbacks
as if they are called unknown number of times" changes logic for
callbacks handling. While previously callbacks were verified as a
single function call, new scheme takes into account that callbacks
could be executed unknown number of times.
This has dire implications for bpf_loop_bench:
SEC("fentry/" SYS_PREFIX "sys_getpgid")
int benchmark(void *ctx)
{
for (int i = 0; i < 1000; i++) {
bpf_loop(nr_loops, empty_callback, NULL, 0);
__sync_add_and_fetch(&hits, nr_loops);
}
return 0;
}
W/o callbacks change verifier sees it as a 1000 calls to
empty_callback(). However, with callbacks change things become
exponential:
- i=0: state exploring empty_callback is scheduled with i=0 (a);
- i=1: state exploring empty_callback is scheduled with i=1;
...
- i=999: state exploring empty_callback is scheduled with i=999;
- state (a) is popped from stack;
- i=1: state exploring empty_callback is scheduled with i=1;
...
Avoid this issue by rewriting outer loop as bpf_loop().
Unfortunately, this adds a function call to a loop at runtime, which
negatively affects performance:
throughput latency
before: 149.919 ± 0.168 M ops/s, 6.670 ns/op
after : 137.040 ± 0.187 M ops/s, 7.297 ns/op
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
30 lines
545 B
C
30 lines
545 B
C
// SPDX-License-Identifier: GPL-2.0
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/* Copyright (c) 2021 Facebook */
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#include "vmlinux.h"
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#include <bpf/bpf_helpers.h>
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#include "bpf_misc.h"
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char _license[] SEC("license") = "GPL";
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u32 nr_loops;
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long hits;
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static int empty_callback(__u32 index, void *data)
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{
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return 0;
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}
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static int outer_loop(__u32 index, void *data)
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{
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bpf_loop(nr_loops, empty_callback, NULL, 0);
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__sync_add_and_fetch(&hits, nr_loops);
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return 0;
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}
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SEC("fentry/" SYS_PREFIX "sys_getpgid")
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int benchmark(void *ctx)
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{
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bpf_loop(1000, outer_loop, NULL, 0);
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return 0;
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}
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