[PATCH v3 4/4] selftests/bpf: Adjust wasted entries threshold for ARM64 BRBE

[Puranjay Mohan]

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@xxxxxxxxxx>
---
 .../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);
-- 
2.52.0