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.
Unified · Split
| @@ -1429,7 +1429,7 @@ static inline u64 time_interpolator_get_cycles(unsigned int src) · 1428 unmodified lines | ||
| 1429 | 1429 | } |
| 1430 | 1430 | } |
| 1431 | 1431 | |
| 1432 | −static inline u64 time_interpolator_get_counter(void) | |
| 1432 | +static inline u64 time_interpolator_get_counter(int writelock) | |
| 1433 | 1433 | { |
| 1434 | 1434 | unsigned int src = time_interpolator->source; |
| 1435 | 1435 | |
| @@ -1443,6 +1443,15 @@ static inline u64 time_interpolator_get_counter(void) · 7 unmodified lines | ||
| 1443 | 1443 | now = time_interpolator_get_cycles(src); |
| 1444 | 1444 | if (lcycle && time_after(lcycle, now)) |
| 1445 | 1445 | return lcycle; |
| 1446 | + | |
| 1447 | + /* When holding the xtime write lock, there's no need | |
| 1448 | + * to add the overhead of the cmpxchg. Readers are | |
| 1449 | + * force to retry until the write lock is released. | |
| 1450 | + */ | |
| 1451 | + if (writelock) { | |
| 1452 | + time_interpolator->last_cycle = now; | |
| 1453 | + return now; | |
| 1454 | + } | |
| 1446 | 1455 | /* Keep track of the last timer value returned. The use of cmpxchg here |
| 1447 | 1456 | * will cause contention in an SMP environment. |
| 1448 | 1457 | */ |
| @@ -1456,7 +1465,7 @@ static inline u64 time_interpolator_get_counter(void) · 7 unmodified lines | ||
| 1456 | 1465 | void time_interpolator_reset(void) |
| 1457 | 1466 | { |
| 1458 | 1467 | time_interpolator->offset = 0; |
| 1459 | − time_interpolator->last_counter = time_interpolator_get_counter(); | |
| 1468 | + time_interpolator->last_counter = time_interpolator_get_counter(1); | |
| 1460 | 1469 | } |
| 1461 | 1470 | |
| 1462 | 1471 | #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) · 5 unmodified lines | ||
| 1468 | 1477 | return 0; |
| 1469 | 1478 | |
| 1470 | 1479 | return time_interpolator->offset + |
| 1471 | − GET_TI_NSECS(time_interpolator_get_counter(), time_interpolator); | |
| 1480 | + GET_TI_NSECS(time_interpolator_get_counter(0), time_interpolator); | |
| 1472 | 1481 | } |
| 1473 | 1482 | |
| 1474 | 1483 | #define INTERPOLATOR_ADJUST 65536 |
| @@ -1491,7 +1500,7 @@ static void time_interpolator_update(long delta_nsec) · 16 unmodified lines | ||
| 1491 | 1500 | * and the tuning logic insures that. |
| 1492 | 1501 | */ |
| 1493 | 1502 | |
| 1494 | − counter = time_interpolator_get_counter(); | |
| 1503 | + counter = time_interpolator_get_counter(1); | |
| 1495 | 1504 | offset = time_interpolator->offset + GET_TI_NSECS(counter, time_interpolator); |
| 1496 | 1505 | |
| 1497 | 1506 | if (delta_nsec < 0 || (unsigned long) delta_nsec < offset) |