On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
> This patch allows one attempt for the lock waiter to steal the lock
> when entering the PV slowpath. This helps to reduce the performance
> penalty caused by lock waiter preemption while not having much of
> the downsides of a real unfair lock.
> @@ -415,8 +458,12 @@ static void pv_wait_head(struct qspinlock *lock, struct
> mcs_spinlock *node)
>
> for (;; waitcnt++) {
> for (loop = SPIN_THRESHOLD; loop; loop--) {
> - if (!READ_ONCE(l->locked))
> - return;
> + /*
> + * Try to acquire the lock when it is free.
> + */
> + if (!READ_ONCE(l->locked) &&
> + (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0))
> + goto gotlock;
> cpu_relax();
> }
>
This isn't _once_, this is once per 'wakeup'. And note that interrupts
unrelated to the kick can equally wake the vCPU up.
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