The spinlock and rwlock use a single-element per-cpu array to track
current locks due to performance reason. But this means the key is
always available and it cannot simply account lock stats in the array
because some of them are invalid.
In fact, the contention_end() program in the BPF invalidates the entry
by setting the 'lock' value to 0 instead of deleting the entry for the
hashmap. So it should skip entries with the lock value of 0 in the
account_end_timestamp().
Otherwise, it'd have spurious high contention on an idle machine:
$ sudo perf lock con -ab -Y spinlock sleep 3
contended total wait max wait avg wait type caller
8 4.72 s 1.84 s 590.46 ms spinlock rcu_core+0xc7
8 1.87 s 1.87 s 233.48 ms spinlock process_one_work+0x1b5
2 1.87 s 1.87 s 933.92 ms spinlock worker_thread+0x1a2
3 1.81 s 1.81 s 603.93 ms spinlock tmigr_update_events+0x13c
2 1.72 s 1.72 s 861.98 ms spinlock tick_do_update_jiffies64+0x25
6 42.48 us 13.02 us 7.08 us spinlock futex_q_lock+0x2a
1 13.03 us 13.03 us 13.03 us spinlock futex_wake+0xce
1 11.61 us 11.61 us 11.61 us spinlock rcu_core+0xc7
I don't believe it has contention on a spinlock longer than 1 second.
After this change, it only reports some small contentions.
$ sudo perf lock con -ab -Y spinlock sleep 3
contended total wait max wait avg wait type caller
4 133.51 us 43.29 us 33.38 us spinlock tick_do_update_jiffies64+0x25
4 69.06 us 31.82 us 17.27 us spinlock process_one_work+0x1b5
2 50.66 us 25.77 us 25.33 us spinlock rcu_core+0xc7
1 28.45 us 28.45 us 28.45 us spinlock rcu_core+0xc7
1 24.77 us 24.77 us 24.77 us spinlock tmigr_update_events+0x13c
1 23.34 us 23.34 us 23.34 us spinlock raw_spin_rq_lock_nested+0x15
Fixes: b5711042a1c8 ("perf lock contention: Use per-cpu array map for spinlocks")
Reported-by: Xi Wang <xii@xxxxxxxxxx>
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
tools/perf/util/bpf_lock_contention.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index 4ee54538aba2..a3d40940fb23 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -296,6 +296,9 @@ static void account_end_timestamp(struct lock_contention *con)
goto next;
for (int i = 0; i < total_cpus; i++) {
+ if (cpu_data[i].lock == 0)
+ continue;
+
update_lock_stat(stat_fd, -1, end_ts, aggr_mode,
&cpu_data[i]);
}
--
2.46.0.295.g3b9ea8a38a-goog
