On 04/15/2013 10:37 AM, Waiman Long wrote:
In the __mutex_lock_common() function, an initial entry into the lock slow path will cause two atomic_xchg instructions to be issued. Together with the atomic decrement in the fast path, a total of three atomic read-modify-write instructions will be issued in rapid succession. This can cause a lot of cache bouncing when many tasks are trying to acquire the mutex at the same time. This patch will reduce the number of atomic_xchg instructions used by checking the counter value first before issuing the instruction. The atomic_read() function is just a simple memory read. The atomic_xchg() function, on the other hand, can be up to 2 order of magnitude or even more in cost when compared with atomic_read(). By using atomic_read() to check the value first before calling atomic_xchg(), we can avoid a lot of unnecessary cache coherency traffic. The only downside with this change is that a task on the slow path will have a tiny bit less chance of getting the mutex when competing with another task in the fast path.
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx> Reviewed-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx> -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
