From: Puranjay Mohan <puranjay@kernel•org>
To: bpf@vger•kernel.org
Cc: Puranjay Mohan <puranjay@kernel•org>,
Puranjay Mohan <puranjay12@gmail•com>,
Alexei Starovoitov <ast@kernel•org>,
Daniel Borkmann <daniel@iogearbox•net>,
John Fastabend <john.fastabend@gmail•com>,
Andrii Nakryiko <andrii@kernel•org>,
Martin KaFai Lau <martin.lau@linux•dev>,
Eduard Zingerman <eddyz87@gmail•com>, Song Liu <song@kernel•org>,
Yonghong Song <yonghong.song@linux•dev>,
Will Deacon <will@kernel•org>,
Mark Rutland <mark.rutland@arm•com>,
Catalin Marinas <catalin.marinas@arm•com>,
Leo Yan <leo.yan@arm•com>, Rob Herring <robh@kernel•org>,
Peter Zijlstra <peterz@infradead•org>,
Ingo Molnar <mingo@redhat•com>,
Arnaldo Carvalho de Melo <acme@kernel•org>,
Namhyung Kim <namhyung@kernel•org>,
James Clark <james.clark@linaro•org>,
Ian Rogers <irogers@google•com>,
Adrian Hunter <adrian.hunter@intel•com>,
Shuah Khan <shuah@kernel•org>, Breno Leitao <leitao@debian•org>,
Ravi Bangoria <ravi.bangoria@amd•com>,
Stephane Eranian <eranian@google•com>,
Kumar Kartikeya Dwivedi <memxor@gmail•com>,
Usama Arif <usama.arif@linux•dev>,
linux-arm-kernel@lists•infradead.org,
linux-perf-users@vger•kernel.org,
linux-kselftest@vger•kernel.org, linux-kernel@vger•kernel.org,
kernel-team@meta•com
Subject: [PATCH v4 4/4] selftests/bpf: Adjust wasted entries threshold for ARM64 BRBE
Date: Wed, 27 May 2026 05:12:00 -0700 [thread overview]
Message-ID: <20260527121207.2312181-5-puranjay@kernel.org> (raw)
In-Reply-To: <20260527121207.2312181-1-puranjay@kernel.org>
The get_branch_snapshot test checks that bpf_get_branch_snapshot()
doesn't waste too many branch entries on infrastructure overhead. The
threshold of < 10 was calibrated for x86 where about 7 entries are
wasted.
On ARM64, the BPF trampoline generates more branches than x86,
resulting in about 13 wasted entries. The overhead comes from the BPF
trampoline calling __bpf_prog_enter_recur which on ARM64 makes
out-of-line calls to __rcu_read_lock and generates more conditional
branches than x86:
[#12] bpf_testmod_loop_test+0x40 -> bpf_trampoline_...+0x48
[#11] bpf_trampoline_...+0x68 -> __bpf_prog_enter_recur+0x0
[#10] __bpf_prog_enter_recur+0x20 -> __bpf_prog_enter_recur+0x118
[#09] __bpf_prog_enter_recur+0x154 -> __bpf_prog_enter_recur+0x160
[#08] __bpf_prog_enter_recur+0x164 -> __bpf_prog_enter_recur+0x2c
[#07] __bpf_prog_enter_recur+0x2c -> __rcu_read_lock+0x0
[#06] __rcu_read_lock+0x18 -> __bpf_prog_enter_recur+0x30
[#05] __bpf_prog_enter_recur+0x9c -> __bpf_prog_enter_recur+0xf0
[#04] __bpf_prog_enter_recur+0xf4 -> __bpf_prog_enter_recur+0xa8
[#03] __bpf_prog_enter_recur+0xb8 -> __bpf_prog_enter_recur+0x100
[#02] __bpf_prog_enter_recur+0x114 -> bpf_trampoline_...+0x6c
[#01] bpf_trampoline_...+0x78 -> bpf_prog_...test1+0x0
[#00] bpf_prog_...test1+0x58 -> arm_brbe_snapshot_branch_stack+0x0
Use an architecture-specific threshold of < 14 for ARM64 to accommodate
this overhead while still detecting regressions.
Signed-off-by: Puranjay Mohan <puranjay@kernel•org>
---
.../selftests/bpf/prog_tests/get_branch_snapshot.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/tools/testing/selftests/bpf/prog_tests/get_branch_snapshot.c b/tools/testing/selftests/bpf/prog_tests/get_branch_snapshot.c
index 0394a1156d99..8d1a3480767f 100644
--- a/tools/testing/selftests/bpf/prog_tests/get_branch_snapshot.c
+++ b/tools/testing/selftests/bpf/prog_tests/get_branch_snapshot.c
@@ -116,13 +116,18 @@ void serial_test_get_branch_snapshot(void)
ASSERT_GT(skel->bss->test1_hits, 6, "find_looptest_in_lbr");
- /* Given we stop LBR in software, we will waste a few entries.
+ /* Given we stop LBR/BRBE in software, we will waste a few entries.
* But we should try to waste as few as possible entries. We are at
- * about 7 on x86_64 systems.
- * Add a check for < 10 so that we get heads-up when something
- * changes and wastes too many entries.
+ * about 7 on x86_64 and about 13 on arm64 systems (the arm64 BPF
+ * trampoline generates more branches than x86_64).
+ * Add a check so that we get heads-up when something changes and
+ * wastes too many entries.
*/
+#if defined(__aarch64__)
+ ASSERT_LT(skel->bss->wasted_entries, 14, "check_wasted_entries");
+#else
ASSERT_LT(skel->bss->wasted_entries, 10, "check_wasted_entries");
+#endif
cleanup:
get_branch_snapshot__destroy(skel);
--
2.53.0-Meta
prev parent reply other threads:[~2026-05-27 12:13 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-27 12:11 [PATCH v4 0/4] arm64: Add BRBE support for bpf_get_branch_snapshot() Puranjay Mohan
2026-05-27 12:11 ` [PATCH v4 1/4] perf/core: Fix sched_task callbacks for CPU-wide branch stack events Puranjay Mohan
2026-05-27 12:11 ` [PATCH v4 2/4] perf: Use a union to clear branch entry bitfields Puranjay Mohan
2026-05-27 13:00 ` bot+bpf-ci
2026-05-27 12:11 ` [PATCH v4 3/4] perf/arm64: Add BRBE support for bpf_get_branch_snapshot() Puranjay Mohan
2026-05-27 13:01 ` bot+bpf-ci
2026-05-27 12:12 ` Puranjay Mohan [this message]
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