tree e750143e578503b77a66eff21595b16730506802
parent 9f4a6dce10941380ad7365cc9b0ef038ed56cbf9
author Alex Williamson <[EMAIL PROTECTED]> Wed, 07 Sep 2005 05:17:04 -0700
committer Linus Torvalds <[EMAIL PROTECTED]> Thu, 08 Sep 2005 06:57:24 -0700
[PATCH] optimize writer path in time_interpolator_get_counter()
Christoph Lameter <[EMAIL PROTECTED]>
When using a time interpolator that is susceptible to jitter there's
potentially contention over a cmpxchg used to prevent time from going
backwards. This is unnecessary when the caller holds the xtime write
seqlock as all readers will be blocked from returning until the write is
complete. We can therefore allow writers to insert a new value and exit
rather than fight with CPUs who only hold a reader lock.
Signed-off-by: Alex Williamson <[EMAIL PROTECTED]>
Signed-off-by: Christoph Lameter <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
kernel/timer.c | 17 +++++++++++++----
1 files changed, 13 insertions(+), 4 deletions(-)
diff --git a/kernel/timer.c b/kernel/timer.c
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1429,7 +1429,7 @@ static inline u64 time_interpolator_get_
}
}
-static inline u64 time_interpolator_get_counter(void)
+static inline u64 time_interpolator_get_counter(int writelock)
{
unsigned int src = time_interpolator->source;
@@ -1443,6 +1443,15 @@ static inline u64 time_interpolator_get_
now = time_interpolator_get_cycles(src);
if (lcycle && time_after(lcycle, now))
return lcycle;
+
+ /* When holding the xtime write lock, there's no need
+ * to add the overhead of the cmpxchg. Readers are
+ * force to retry until the write lock is released.
+ */
+ if (writelock) {
+ time_interpolator->last_cycle = now;
+ return now;
+ }
/* Keep track of the last timer value returned. The use
of cmpxchg here
* will cause contention in an SMP environment.
*/
@@ -1456,7 +1465,7 @@ static inline u64 time_interpolator_get_
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(1);
}
#define GET_TI_NSECS(count,i) (((((count) - i->last_counter) & (i)->mask) *
(i)->nsec_per_cyc) >> (i)->shift)
@@ -1468,7 +1477,7 @@ unsigned long time_interpolator_get_offs
return 0;
return time_interpolator->offset +
- GET_TI_NSECS(time_interpolator_get_counter(),
time_interpolator);
+ GET_TI_NSECS(time_interpolator_get_counter(0),
time_interpolator);
}
#define INTERPOLATOR_ADJUST 65536
@@ -1491,7 +1500,7 @@ static void time_interpolator_update(lon
* and the tuning logic insures that.
*/
- counter = time_interpolator_get_counter();
+ counter = time_interpolator_get_counter(1);
offset = time_interpolator->offset + GET_TI_NSECS(counter,
time_interpolator);
if (delta_nsec < 0 || (unsigned long) delta_nsec < offset)
-
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