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@xxxxxxxxxx>
---
.../selftests/bpf/benchs/bench_trigger.c | 40 ++++++++++++++++---
.../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;
}
--
2.43.0
