Ok, here's an optimization that should help reduce contention on the
cmpxchg, has zero impact on the nojitter path, and doesn't require any
changes to fsys. When a caller already had the xtime_lock write lock
there's no need to fight with other CPUs for the cmpxchg. The other
"reader" CPUs will have to fetch it again since a seqlock write is in
progress. Therefore we can simplify this path as shown below. The
write is atomic, and we don't care if another CPU has changed last_cycle
since it can't return the value until the write lock is released. This
has only been compile tested, but I'm interested to hear your opinion.
Thanks,
Alex
diff -r cff8d3633e9c kernel/timer.c
--- a/kernel/timer.c Fri Jul 29 22:01:15 2005
+++ b/kernel/timer.c Sun Jul 31 10:25:41 2005
@@ -1452,10 +1452,34 @@
return time_interpolator_get_cycles(src);
}
+static inline u64 time_interpolator_get_counter_locked(void)
+{
+ unsigned int src = time_interpolator->source;
+
+ if (time_interpolator->jitter)
+ {
+ u64 lcycle = time_interpolator->last_cycle;
+ u64 now = time_interpolator_get_cycles(src);
+
+ if (lcycle && time_after(lcycle, now))
+ return lcycle;
+
+ /*
+ * This path is called when holding the xtime write lock.
+ * This allows us to avoid the contention of the cmpxchg
+ * in get_counter, and still ensures jitter protection.
+ */
+ time_interpolator->last_cycle = now;
+ return now;
+ }
+ else
+ return time_interpolator_get_cycles(src);
+}
+
void time_interpolator_reset(void)
{
time_interpolator->offset = 0;
- time_interpolator->last_counter = time_interpolator_get_counter();
+ time_interpolator->last_counter = time_interpolator_get_counter_locked();
}
#define GET_TI_NSECS(count,i) (((((count) - i->last_counter) & (i)->mask) * (i)->nsec_per_cyc) >> (i)->shift)
@@ -1490,7 +1514,7 @@
* and the tuning logic insures that.
*/
- counter = time_interpolator_get_counter();
+ counter = time_interpolator_get_counter_locked();
offset = time_interpolator->offset + GET_TI_NSECS(counter, time_interpolator);
if (delta_nsec < 0 || (unsigned long) delta_nsec < offset)
-
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