linux-kernel - [PATCH rcu 4/4] smp: print only local CPU info when sched

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Message-Id: <20240816043917.26537-4-neeraj.upadhyay@kernel.org>
Date: Fri, 16 Aug 2024 10:09:17 +0530
From: neeraj.upadhyay@...nel.org
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	paulmck@...nel.org,
	neeraj.upadhyay@...nel.org,
	neeraj.upadhyay@....com,
	boqun.feng@...il.com,
	joel@...lfernandes.org,
	urezki@...il.com,
	frederic@...nel.org,
	mingo@...nel.org,
	peterz@...radead.org,
	leobras@...hat.com,
	imran.f.khan@...cle.com,
	riel@...riel.com,
	tglx@...utronix.de
Subject: [PATCH rcu 4/4] smp: print only local CPU info when sched_clock goes backward

From: Rik van Riel <riel@...riel.com>

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 <riel@...riel.com>
Tested-by: "Paul E. McKenney" <paulmck@...nel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@...nel.org>
---
 kernel/smp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/smp.c b/kernel/smp.c
index b484ee6dcaf6..f25e20617b7e 100644
--- 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);
-- 
2.40.1