smp: print only local CPU info when sched_clock goes backward

About 40% of all csd_lock warnings observed in our fleet appear to be due to sched_clock() going backward in time (usually only a little bit), resulting in ts0 being larger than ts2. When the local CPU is at fault, we should print out a message reflecting that, rather than trying to get the remote CPU's stack trace. Signed-off-by: Rik van Riel <[email protected]> Tested-by: "Paul E. McKenney" <[email protected]> Signed-off-by: Neeraj Upadhyay <[email protected]>
pop-os · Aug 14, 2024 · 9fbaa44 · 9fbaa44
1 parent d40760d
commit 9fbaa44
Showing 1 changed file with 8 additions and 0 deletions.
diff --git a/kernel/smp.c b/kernel/smp.c
@@ -254,6 +254,14 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 		   csd_lock_timeout_ns == 0))
 		return false;
 
+	if (ts0 > ts2) {
+		/* Our own sched_clock went backward; don't blame another CPU. */
+		ts_delta = ts0 - ts2;
+		pr_alert("sched_clock on CPU %d went backward by %llu ns\n", raw_smp_processor_id(), ts_delta);
+		*ts1 = ts2;
+		return false;
+	}
+
 	firsttime = !*bug_id;
 	if (firsttime)
 		*bug_id = atomic_inc_return(&csd_bug_count);