Techwizz/kernel-fxtec-pro1x
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

[PATCH] optimize writer path in time_interpolator_get_counter()

Browse source 
Christoph Lameter <clameter@engr.sgi.com>

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 <alex.williamson@hp.com>
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Alex Williamson 2005-09-06 15:17:04 -07:00 committed by Linus Torvalds
parent 9f4a6dce10
commit 486d46aefe
1 changed files with 13 additions and 4 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

17
kernel/timer.c
View file

@ -1429,7 +1429,7 @@ static inline u64 time_interpolator_get_cycles(unsigned int src)
} }
} }
static inline u64 time_interpolator_get_counter(void) static inline u64 time_interpolator_get_counter(int writelock)
{ {
unsigned int src = time_interpolator->source; unsigned int src = time_interpolator->source;
@ -1443,6 +1443,15 @@ static inline u64 time_interpolator_get_counter(void)
now = time_interpolator_get_cycles(src); now = time_interpolator_get_cycles(src);
if (lcycle && time_after(lcycle, now)) if (lcycle && time_after(lcycle, now))
return lcycle; 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 /* Keep track of the last timer value returned. The use of cmpxchg here
* will cause contention in an SMP environment. * will cause contention in an SMP environment.
*/ */
@ -1456,7 +1465,7 @@ static inline u64 time_interpolator_get_counter(void)
void time_interpolator_reset(void) void time_interpolator_reset(void)
{ {
time_interpolator->offset = 0; 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) #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_offset(void)
return 0; return 0;
return time_interpolator->offset + 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 #define INTERPOLATOR_ADJUST 65536
@ -1491,7 +1500,7 @@ static void time_interpolator_update(long delta_nsec)
* and the tuning logic insures that. * 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); offset = time_interpolator->offset + GET_TI_NSECS(counter, time_interpolator);
if (delta_nsec < 0 || (unsigned long) delta_nsec < offset) if (delta_nsec < 0 || (unsigned long) delta_nsec < offset)