timekeeping: optimized ktime_get[_ts] for GENERIC_TIME=y

The generic ktime_get function defined in kernel/hrtimer.c is suboptimial
for GENERIC_TIME=y:

 0)               |  ktime_get() {
 0)               |    ktime_get_ts() {
 0)               |      getnstimeofday() {
 0)               |        read_tod_clock() {
 0)   0.601 us    |        }
 0)   1.938 us    |      }
 0)               |      set_normalized_timespec() {
 0)   0.602 us    |      }
 0)   4.375 us    |    }
 0)   5.523 us    |  }

Overall there are two read_seqbegin/read_seqretry loops and a lot of
unnecessary struct timespec calculations. ktime_get returns a nano second
value which is the sum of xtime, wall_to_monotonic and the nano second
delta from the clock source.

ktime_get can be optimized for GENERIC_TIME=y. The new version only calls
clocksource_read:

 0)               |  ktime_get() {
 0)               |    read_tod_clock() {
 0)   0.610 us    |    }
 0)   1.977 us    |  }

It uses a single read_seqbegin/readseqretry loop and just adds everthing
to a nano second value.

ktime_get_ts is optimized in a similar fashion.

[ tglx: added WARN_ON(timekeeping_suspended) as in getnstimeofday() ]

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: john stultz <johnstul@us.ibm.com>
LKML-Reference: <20090707112728.3005244d@skybase>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 69 insertions, 0 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e8c77d9c633a..7a248135c6f2 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -125,6 +125,75 @@ void getnstimeofday(struct timespec *ts)
 
 EXPORT_SYMBOL(getnstimeofday);
 
+ktime_t ktime_get(void)
+{
+        cycle_t cycle_now, cycle_delta;
+        unsigned int seq;
+        s64 secs, nsecs;
+
+        WARN_ON(timekeeping_suspended);
+
+        do {
+                seq = read_seqbegin(&xtime_lock);
+                secs = xtime.tv_sec + wall_to_monotonic.tv_sec;
+                nsecs = xtime.tv_nsec + wall_to_monotonic.tv_nsec;
+
+                /* read clocksource: */
+                cycle_now = clocksource_read(clock);
+
+                /* calculate the delta since the last update_wall_time: */
+                cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
+
+                /* convert to nanoseconds: */
+                nsecs += cyc2ns(clock, cycle_delta);
+
+        } while (read_seqretry(&xtime_lock, seq));
+        /*
+         * Use ktime_set/ktime_add_ns to create a proper ktime on
+         * 32-bit architectures without CONFIG_KTIME_SCALAR.
+         */
+        return ktime_add_ns(ktime_set(secs, 0), nsecs);
+}
+EXPORT_SYMBOL_GPL(ktime_get);
+
+/**
+ * ktime_get_ts - get the monotonic clock in timespec format
+ * @ts:         pointer to timespec variable
+ *
+ * The function calculates the monotonic clock from the realtime
+ * clock and the wall_to_monotonic offset and stores the result
+ * in normalized timespec format in the variable pointed to by @ts.
+ */
+void ktime_get_ts(struct timespec *ts)
+{
+        cycle_t cycle_now, cycle_delta;
+        struct timespec tomono;
+        unsigned int seq;
+        s64 nsecs;
+
+        WARN_ON(timekeeping_suspended);
+
+        do {
+                seq = read_seqbegin(&xtime_lock);
+                *ts = xtime;
+                tomono = wall_to_monotonic;
+
+                /* read clocksource: */
+                cycle_now = clocksource_read(clock);
+
+                /* calculate the delta since the last update_wall_time: */
+                cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
+
+                /* convert to nanoseconds: */
+                nsecs = cyc2ns(clock, cycle_delta);
+
+        } while (read_seqretry(&xtime_lock, seq));
+
+        set_normalized_timespec(ts, ts->tv_sec + tomono.tv_sec,
+                                ts->tv_nsec + tomono.tv_nsec + nsecs);
+}
+EXPORT_SYMBOL_GPL(ktime_get_ts);
+
 /**
  * do_gettimeofday - Returns the time of day in a timeval
  * @tv:         pointer to the timeval to be set