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

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Message-ID: <20240715134941.7ac59eb9@imladris.surriel.com>
Date: Mon, 15 Jul 2024 13:49:41 -0400
From: Rik van Riel <riel@...riel.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 linux-kernel@...r.kernel.org, kernel-team@...a.com,
 neeraj.upadhyay@...nel.org, mingo@...nel.org, peterz@...radead.org,
 rostedt@...dmis.org, Leonardo Bras <leobras@...hat.com>
Subject: [PATCH] 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 <riel@...riel.com>
---
 kernel/smp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/smp.c b/kernel/smp.c
index f085ebcdf9e7..5656ef63ea82 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -237,6 +237,14 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 	if (likely(ts_delta <= csd_lock_timeout_ns || 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);
-- 
2.45.2