Messages in this thread | | | Date | Mon, 14 Sep 2015 15:38:32 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt |
| |
On 09/14/2015 03:15 PM, Waiman Long wrote: > On 09/14/2015 10:00 AM, Peter Zijlstra wrote: >> 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. >> > > Oh! There is a minor bug that I shouldn't need to have a second > READ_ONCE() call here.
Oh! I misread the diff, the code was OK.
Cheers, Longman
| |