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 b484ee6dcaf6fcb51a268d7219a560d0f4cb83b7..f25e20617b7eb7a507b002b18672ab197f2ee8e5 100644 (file)
--- 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);