[PATCH v2 2/3] locking/csd-lock: Report how long a stuck CSD lock took to recover

[Breno Leitao <leitao@debian.org>]

The CSD lock debug output is a useful way to catch IPI stalls, but when
the lock finally recovers it only says that it did:

  smp: csd: CSD lock (#1) got unstuck on CPU#32, CPU#123 released the lock.

How long the target took to answer is left out, even though
csd_lock_wait_toolong() already has the timestamp the wait started from.

At Meta's fleet, that line fired 211K times in the last 24 hours, so
plenty of stalls get reported with no indication of how long they
lasted.

Print how long the lock was stuck, and, when an IPI was re-sent, how
long after that re-send the target released, as:

  smp: csd: CSD lock (#1) got unstuck on CPU#00, CPU#01 released the lock after 8000854660 ns, 3000825778 ns after the last IPI re-send.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>
---
 kernel/smp.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index e00b8f620c5d9..c7475b3deb574 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -227,10 +227,28 @@ bool csd_lock_is_stuck(void)
 struct csd_wait_state {
 	u64 ts_start;		/* When the wait began. */
 	u64 ts_report;		/* When the last complaint was printed. */
+	u64 ts_resend;		/* When the last IPI was re-sent, 0 if never. */
 	int bug_id;
 	unsigned long nmessages;
 };
 
+/*
+ * Report a CSD lock that came back, @ts_unstuck being when the release was
+ * noticed.  Only mention the re-send delta if an IPI was actually re-sent.
+ */
+static void csd_lock_print_unstuck(struct csd_wait_state *state, int cpu, u64 ts_unstuck)
+{
+	if (state->ts_resend)
+		pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, CPU#%02d released the lock after %lld ns, %lld ns after the last IPI re-send.\n",
+			 state->bug_id, raw_smp_processor_id(), cpu,
+			 (s64)(ts_unstuck - state->ts_start),
+			 (s64)(ts_unstuck - state->ts_resend));
+	else
+		pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, CPU#%02d released the lock after %lld ns.\n",
+			 state->bug_id, raw_smp_processor_id(), cpu,
+			 (s64)(ts_unstuck - state->ts_start));
+}
+
 /*
  * Complain if too much time spent waiting.  Note that only
  * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
@@ -249,9 +267,9 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, struct csd_wait_state
 	if (!(flags & CSD_FLAG_LOCK)) {
 		if (!unlikely(state->bug_id))
 			return true;
+		ts_now = ktime_get_mono_fast_ns();
 		cpu = csd_lock_wait_getcpu(csd);
-		pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, CPU#%02d released the lock.\n",
-			 state->bug_id, raw_smp_processor_id(), cpu);
+		csd_lock_print_unstuck(state, cpu, ts_now);
 		atomic_dec(&n_csd_lock_stuck);
 		return true;
 	}
@@ -309,6 +327,7 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, struct csd_wait_state
 		if (!cpu_cur_csd) {
 			pr_alert("csd: Re-sending CSD lock (#%d) IPI from CPU#%02d to CPU#%02d\n", state->bug_id, raw_smp_processor_id(), cpu);
 			arch_send_call_function_single_ipi(cpu);
+			state->ts_resend = ktime_get_mono_fast_ns();
 		}
 	}
 	if (firsttime)

-- 
2.53.0-Meta