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SubjectRe: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning
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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