Messages in this thread | | | Date | Mon, 09 Nov 2015 11:51:20 -0500 | From | Waiman Long <> | Subject | Re: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning |
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On 11/06/2015 03:37 PM, Peter Zijlstra wrote: > On Fri, Nov 06, 2015 at 12:54:06PM -0500, Waiman Long wrote: >>>> +static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev) >>>> { >>>> struct pv_node *pn = (struct pv_node *)node; >>>> + struct pv_node *pp = (struct pv_node *)prev; >>>> int waitcnt = 0; >>>> int loop; >>>> + bool wait_early; >>>> >>>> /* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */ >>>> for (;; waitcnt++) { >>>> - for (loop = SPIN_THRESHOLD; loop; loop--) { >>>> + for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) { >>>> if (READ_ONCE(node->locked)) >>>> return; >>>> + if (pv_wait_early(pp, loop)) { >>>> + wait_early = true; >>>> + break; >>>> + } >>>> cpu_relax(); >>>> } >>>> >>> So if prev points to another node, it will never see vcpu_running. Was >>> that fully intended? >> I had added code in pv_wait_head_or_lock to set the state appropriately for >> the queue head vCPU. > Yes, but that's the head, for nodes we'll always have halted or hashed.
The node state was initialized to be vcpu_running. In pv_wait_node(), it will be changed to vcpu_halted before sleeping and back to vcpu_running after that. So it is not true that it is either halted or hashed.
In case, it was changed to vcpu_hashed, it will be changed back to vcpu_running in pv_wait_head_lock before entering the active spinning loop. There are definitely a small amount of time where the node state does not reflect the actual vCPU state, but that is the best we can do so far.
Cheers, Longman
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