locking/csd-lock: Switch from sched_clock() to ktime_get_mono_fast_ns()

Currently, the CONFIG_CSD_LOCK_WAIT_DEBUG code uses sched_clock() to check
for excessive CSD-lock wait times.  This works, but does not guarantee
monotonic timestamps on x86 due to the sched_clock() function's use of
the rdtsc instruction, which does not guarantee ordering.  This means
that, given successive calls to sched_clock(), the second might return
an earlier time than the second, that is, time might seem to go backwards.
This can (and does!) result in false-positive CSD-lock wait complaints
claiming almost 2^64 nanoseconds of delay.

Therefore, switch from sched_clock() to ktime_get_mono_fast_ns(), which
does guarantee monotonic timestamps via the rdtsc_ordered() function,
which as the name implies, does guarantee ordered timestamps, at least
in the absence of calls from NMI handlers, which are not involved in
this code path.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>

diff --git a/kernel/smp.c b/kernel/smp.c
index f25e20617b7eb7a507b002b18672ab197f2ee8e5..27dc31a146a3599564e46ca55766c7e9dca8ca35 100644 (file)
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -246,7 +246,7 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
                return true;
        }
 
-       ts2 = sched_clock();
+       ts2 = ktime_get_mono_fast_ns();
        /* How long since we last checked for a stuck CSD lock.*/
        ts_delta = ts2 - *ts1;
        if (likely(ts_delta <= csd_lock_timeout_ns * (*nmessages + 1) *
@@ -321,7 +321,7 @@ static void __csd_lock_wait(call_single_data_t *csd)
        int bug_id = 0;
        u64 ts0, ts1;
 
-       ts1 = ts0 = sched_clock();
+       ts1 = ts0 = ktime_get_mono_fast_ns();
        for (;;) {
                if (csd_lock_wait_toolong(csd, ts0, &ts1, &bug_id, &nmessages))
                        break;