From: Rik van Riel <riel@surriel.com>
Date: Mon, 15 Jul 2024 17:49:41 +0000 (-0400)
Subject: smp: print only local CPU info when sched_clock goes backward
X-Git-Tag: v6.12-rc1~166^2^4~5
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9fbaa44114ca6b69074a488295e86a9fa7e685f9;p=thirdparty%2Flinux.git

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@surriel.com>
Tested-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
---

diff --git a/kernel/smp.c b/kernel/smp.c
index b484ee6dcaf6f..f25e20617b7eb 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);