[PATCH 2/2] perf lock contention: Show detail failure reason for BPF

[Namhyung Kim <namhyung@kernel.org>]

It can fail to collect lock stat from BPF for various reasons.  For
example, I've got a report that sometimes time calculation seems wrong
in case of contended spinlocks.  I suspect the time delta went negative
for some reason.

Count them separately and show in the output like below:

$ sudo perf lock contention -abE5 sleep 10
 contended   total wait     max wait     avg wait         type   caller

        13    785.61 us     79.36 us     60.43 us     spinlock   remove_wait_queue+0x14
        10    469.02 us     87.51 us     46.90 us     spinlock   prepare_to_wait+0x27
         9    289.09 us     69.08 us     32.12 us     spinlock   finish_wait+0x36
       114    251.05 us      8.56 us      2.20 us     spinlock   try_to_wake_up+0x1f5
       132    188.63 us      5.01 us      1.43 us     spinlock   __wake_up_common_lock+0x62

=== output for debug ===

bad: 1, total: 279
bad rate: 0.36 %
histogram of failure reasons
       task: 1
      stack: 0
       time: 0

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/builtin-lock.c                     | 28 +++++++++++++++----
 tools/perf/util/bpf_lock_contention.c         |  4 ++-
 .../perf/util/bpf_skel/lock_contention.bpf.c  | 13 +++++++--
 tools/perf/util/lock-contention.h             |  8 +++++-
 4 files changed, 42 insertions(+), 11 deletions(-)

diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
index 82dd2dfe5692..32ec58fb80e4 100644
--- a/tools/perf/builtin-lock.c
+++ b/tools/perf/builtin-lock.c
@@ -1619,6 +1619,24 @@ static void sort_contention_result(void)
 	sort_result();
 }
 
+static void print_bpf_events(int total, struct lock_contention_fails *fails)
+{
+	/* Output for debug, this have to be removed */
+	int broken = fails->task + fails->stack + fails->time;
+
+	if (quiet || total == 0 || (broken == 0 && verbose <= 0))
+		return;
+
+	total += broken;
+	pr_info("\n=== output for debug ===\n\n");
+	pr_info("bad: %d, total: %d\n", broken, total);
+	pr_info("bad rate: %.2f %%\n", (double)broken / (double)total * 100);
+
+	pr_info("histogram of failure reasons\n");
+	pr_info(" %10s: %d\n", "task", fails->task);
+	pr_info(" %10s: %d\n", "stack", fails->stack);
+	pr_info(" %10s: %d\n", "time", fails->time);
+}
 static void print_contention_result(struct lock_contention *con)
 {
 	struct lock_stat *st;
@@ -1646,8 +1664,6 @@ static void print_contention_result(struct lock_contention *con)
 	}
 
 	bad = total = printed = 0;
-	if (use_bpf)
-		bad = bad_hist[BROKEN_CONTENDED];
 
 	while ((st = pop_from_result())) {
 		struct thread *t;
@@ -1704,7 +1720,10 @@ static void print_contention_result(struct lock_contention *con)
 			break;
 	}
 
-	print_bad_events(bad, total);
+	if (use_bpf)
+		print_bpf_events(total, &con->fails);
+	else
+		print_bad_events(bad, total);
 }
 
 static bool force;
@@ -1931,9 +1950,6 @@ static int __cmd_contention(int argc, const char **argv)
 
 		lock_contention_stop();
 		lock_contention_read(&con);
-
-		/* abuse bad hist stats for lost entries */
-		bad_hist[BROKEN_CONTENDED] = con.lost;
 	} else {
 		err = perf_session__process_events(session);
 		if (err)
diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index 5927bf0bd92b..b3e8aa9cd011 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -259,7 +259,9 @@ int lock_contention_read(struct lock_contention *con)
 	fd = bpf_map__fd(skel->maps.lock_stat);
 	stack = bpf_map__fd(skel->maps.stacks);
 
-	con->lost = skel->bss->lost;
+	con->fails.task = skel->bss->task_fail;
+	con->fails.stack = skel->bss->stack_fail;
+	con->fails.time = skel->bss->time_fail;
 
 	stack_trace = zalloc(stack_size);
 	if (stack_trace == NULL)
diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index ed9160999c32..3f4ee3992e81 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -124,7 +124,9 @@ int lock_owner;
 int aggr_mode;
 
 /* error stat */
-int lost;
+int task_fail;
+int stack_fail;
+int time_fail;
 
 static inline int can_record(u64 *ctx)
 {
@@ -283,7 +285,7 @@ int contention_begin(u64 *ctx)
 		bpf_map_update_elem(&tstamp, &pid, &zero, BPF_ANY);
 		pelem = bpf_map_lookup_elem(&tstamp, &pid);
 		if (pelem == NULL) {
-			lost++;
+			__sync_fetch_and_add(&task_fail, 1);
 			return 0;
 		}
 	}
@@ -296,7 +298,7 @@ int contention_begin(u64 *ctx)
 		pelem->stack_id = bpf_get_stackid(ctx, &stacks,
 						  BPF_F_FAST_STACK_CMP | stack_skip);
 		if (pelem->stack_id < 0)
-			lost++;
+			__sync_fetch_and_add(&stack_fail, 1);
 	} else if (aggr_mode == LOCK_AGGR_TASK) {
 		struct task_struct *task;
 
@@ -340,6 +342,11 @@ int contention_end(u64 *ctx)
 		return 0;
 
 	duration = bpf_ktime_get_ns() - pelem->timestamp;
+	if ((__s64)duration < 0) {
+		bpf_map_delete_elem(&tstamp, &pid);
+		__sync_fetch_and_add(&time_fail, 1);
+		return 0;
+	}
 
 	switch (aggr_mode) {
 	case LOCK_AGGR_CALLER:
diff --git a/tools/perf/util/lock-contention.h b/tools/perf/util/lock-contention.h
index 040b618b2215..10c28302420c 100644
--- a/tools/perf/util/lock-contention.h
+++ b/tools/perf/util/lock-contention.h
@@ -122,14 +122,20 @@ struct evlist;
 struct machine;
 struct target;
 
+struct lock_contention_fails {
+	int task;
+	int stack;
+	int time;
+};
+
 struct lock_contention {
 	struct evlist *evlist;
 	struct target *target;
 	struct machine *machine;
 	struct hlist_head *result;
 	struct lock_filter *filters;
+	struct lock_contention_fails fails;
 	unsigned long map_nr_entries;
-	int lost;
 	int max_stack;
 	int stack_skip;
 	int aggr_mode;
-- 
2.40.0.348.gf938b09366-goog