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author | Andrii Nakryiko <andrii@kernel.org> | 2024-03-16 00:33:29 +0300 |
---|---|---|
committer | Alexei Starovoitov <ast@kernel.org> | 2024-03-20 09:41:35 +0300 |
commit | 520fad2e3206b2476f201a283ecf096cfb671902 (patch) | |
tree | e4fee37094eff46816887a0f211fe37972ce71f5 | |
parent | e9a826dd145bf2c19888aee1b974214cefc74a2e (diff) | |
download | linux-520fad2e3206b2476f201a283ecf096cfb671902.tar.xz |
selftests/bpf: scale benchmark counting by using per-CPU counters
When benchmarking with multiple threads (-pN, where N>1), we start
contending on single atomic counter that both BPF trigger benchmarks are
using, as well as "baseline" tests in user space (trig-base and
trig-uprobe-base benchmarks). As such, we start bottlenecking on
something completely irrelevant to benchmark at hand.
Scale counting up by using per-CPU counters on BPF side. On use space
side we do the next best thing: hash thread ID to approximate per-CPU
behavior. It seems to work quite well in practice.
To demonstrate the difference, I ran three benchmarks with 1, 2, 4, 8,
16, and 32 threads:
- trig-uprobe-base (no syscalls, pure tight counting loop in user-space);
- trig-base (get_pgid() syscall, atomic counter in user-space);
- trig-fentry (syscall to trigger fentry program, atomic uncontended per-CPU
counter on BPF side).
Command used:
for b in uprobe-base base fentry; do \
for p in 1 2 4 8 16 32; do \
printf "%-11s %2d: %s\n" $b $p \
"$(sudo ./bench -w2 -d5 -a -p$p trig-$b | tail -n1 | cut -d'(' -f1 | cut -d' ' -f3-)"; \
done; \
done
Before these changes, aggregate throughput across all threads doesn't
scale well with number of threads, it actually even falls sharply for
uprobe-base due to a very high contention:
uprobe-base 1: 138.998 ± 0.650M/s
uprobe-base 2: 70.526 ± 1.147M/s
uprobe-base 4: 63.114 ± 0.302M/s
uprobe-base 8: 54.177 ± 0.138M/s
uprobe-base 16: 45.439 ± 0.057M/s
uprobe-base 32: 37.163 ± 0.242M/s
base 1: 16.940 ± 0.182M/s
base 2: 19.231 ± 0.105M/s
base 4: 21.479 ± 0.038M/s
base 8: 23.030 ± 0.037M/s
base 16: 22.034 ± 0.004M/s
base 32: 18.152 ± 0.013M/s
fentry 1: 14.794 ± 0.054M/s
fentry 2: 17.341 ± 0.055M/s
fentry 4: 23.792 ± 0.024M/s
fentry 8: 21.557 ± 0.047M/s
fentry 16: 21.121 ± 0.004M/s
fentry 32: 17.067 ± 0.023M/s
After these changes, we see almost perfect linear scaling, as expected.
The sub-linear scaling when going from 8 to 16 threads is interesting
and consistent on my test machine, but I haven't investigated what is
causing it this peculiar slowdown (across all benchmarks, could be due
to hyperthreading effects, not sure).
uprobe-base 1: 139.980 ± 0.648M/s
uprobe-base 2: 270.244 ± 0.379M/s
uprobe-base 4: 532.044 ± 1.519M/s
uprobe-base 8: 1004.571 ± 3.174M/s
uprobe-base 16: 1720.098 ± 0.744M/s
uprobe-base 32: 3506.659 ± 8.549M/s
base 1: 16.869 ± 0.071M/s
base 2: 33.007 ± 0.092M/s
base 4: 64.670 ± 0.203M/s
base 8: 121.969 ± 0.210M/s
base 16: 207.832 ± 0.112M/s
base 32: 424.227 ± 1.477M/s
fentry 1: 14.777 ± 0.087M/s
fentry 2: 28.575 ± 0.146M/s
fentry 4: 56.234 ± 0.176M/s
fentry 8: 106.095 ± 0.385M/s
fentry 16: 181.440 ± 0.032M/s
fentry 32: 369.131 ± 0.693M/s
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Message-ID: <20240315213329.1161589-1-andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
-rw-r--r-- | tools/testing/selftests/bpf/benchs/bench_trigger.c | 40 | ||||
-rw-r--r-- | tools/testing/selftests/bpf/progs/trigger_bench.c | 39 |
2 files changed, 62 insertions, 17 deletions
diff --git a/tools/testing/selftests/bpf/benchs/bench_trigger.c b/tools/testing/selftests/bpf/benchs/bench_trigger.c index ace0d1011a8e..b7aea79495ba 100644 --- a/tools/testing/selftests/bpf/benchs/bench_trigger.c +++ b/tools/testing/selftests/bpf/benchs/bench_trigger.c @@ -1,15 +1,45 @@ // SPDX-License-Identifier: GPL-2.0 /* Copyright (c) 2020 Facebook */ +#define _GNU_SOURCE +#include <unistd.h> #include "bench.h" #include "trigger_bench.skel.h" #include "trace_helpers.h" +/* adjust slot shift in inc_hits() if changing */ +#define MAX_BUCKETS 256 + /* BPF triggering benchmarks */ static struct trigger_ctx { struct trigger_bench *skel; } ctx; -static struct counter base_hits; +static struct counter base_hits[MAX_BUCKETS]; + +static __always_inline void inc_counter(struct counter *counters) +{ + static __thread int tid = 0; + unsigned slot; + + if (unlikely(tid == 0)) + tid = gettid(); + + /* multiplicative hashing, it's fast */ + slot = 2654435769U * tid; + slot >>= 24; + + atomic_inc(&base_hits[slot].value); /* use highest byte as an index */ +} + +static long sum_and_reset_counters(struct counter *counters) +{ + int i; + long sum = 0; + + for (i = 0; i < MAX_BUCKETS; i++) + sum += atomic_swap(&counters[i].value, 0); + return sum; +} static void trigger_validate(void) { @@ -23,14 +53,14 @@ static void *trigger_base_producer(void *input) { while (true) { (void)syscall(__NR_getpgid); - atomic_inc(&base_hits.value); + inc_counter(base_hits); } return NULL; } static void trigger_base_measure(struct bench_res *res) { - res->hits = atomic_swap(&base_hits.value, 0); + res->hits = sum_and_reset_counters(base_hits); } static void *trigger_producer(void *input) @@ -42,7 +72,7 @@ static void *trigger_producer(void *input) static void trigger_measure(struct bench_res *res) { - res->hits = atomic_swap(&ctx.skel->bss->hits, 0); + res->hits = sum_and_reset_counters(ctx.skel->bss->hits); } static void setup_ctx(void) @@ -164,7 +194,7 @@ static void *uprobe_base_producer(void *input) { while (true) { uprobe_target_nop(); - atomic_inc(&base_hits.value); + inc_counter(base_hits); } return NULL; } diff --git a/tools/testing/selftests/bpf/progs/trigger_bench.c b/tools/testing/selftests/bpf/progs/trigger_bench.c index 5fda43901033..42ec202015ed 100644 --- a/tools/testing/selftests/bpf/progs/trigger_bench.c +++ b/tools/testing/selftests/bpf/progs/trigger_bench.c @@ -9,12 +9,27 @@ char _license[] SEC("license") = "GPL"; -long hits = 0; +#define CPU_MASK 255 +#define MAX_CPUS (CPU_MASK + 1) /* should match MAX_BUCKETS in benchs/bench_trigger.c */ + +/* matches struct counter in bench.h */ +struct counter { + long value; +} __attribute__((aligned(128))); + +struct counter hits[MAX_CPUS]; + +static __always_inline void inc_counter(void) +{ + int cpu = bpf_get_smp_processor_id(); + + __sync_add_and_fetch(&hits[cpu & CPU_MASK].value, 1); +} SEC("tp/syscalls/sys_enter_getpgid") int bench_trigger_tp(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } @@ -22,69 +37,69 @@ SEC("raw_tp/sys_enter") int BPF_PROG(bench_trigger_raw_tp, struct pt_regs *regs, long id) { if (id == __NR_getpgid) - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("kprobe/" SYS_PREFIX "sys_getpgid") int bench_trigger_kprobe(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("kretprobe/" SYS_PREFIX "sys_getpgid") int bench_trigger_kretprobe(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("kprobe.multi/" SYS_PREFIX "sys_getpgid") int bench_trigger_kprobe_multi(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("kretprobe.multi/" SYS_PREFIX "sys_getpgid") int bench_trigger_kretprobe_multi(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("fentry/" SYS_PREFIX "sys_getpgid") int bench_trigger_fentry(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("fexit/" SYS_PREFIX "sys_getpgid") int bench_trigger_fexit(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("fentry.s/" SYS_PREFIX "sys_getpgid") int bench_trigger_fentry_sleep(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } SEC("fmod_ret/" SYS_PREFIX "sys_getpgid") int bench_trigger_fmodret(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return -22; } SEC("uprobe") int bench_trigger_uprobe(void *ctx) { - __sync_add_and_fetch(&hits, 1); + inc_counter(); return 0; } |