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SubjectRe: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning
On 11/06/2015 10:01 AM, Peter Zijlstra wrote:
> On Fri, Oct 30, 2015 at 07:26:37PM -0400, Waiman Long wrote:
>> +++ b/kernel/locking/qspinlock_paravirt.h
>> @@ -23,6 +23,19 @@
>> #define _Q_SLOW_VAL (3U<< _Q_LOCKED_OFFSET)
>>
>> /*
>> + * Queue Node Adaptive Spinning
>> + *
>> + * A queue node vCPU will stop spinning if the vCPU in the previous node is
>> + * not running. The one lock stealing attempt allowed at slowpath entry
>> + * mitigates the slight slowdown for non-overcommitted guest with this
>> + * aggressive wait-early mechanism.
>> + *
>> + * The status of the previous node will be checked at fixed interval
>> + * controlled by PV_PREV_CHECK_MASK.
>> + */
>> +#define PV_PREV_CHECK_MASK 0xff
>> +
>> +/*
>> * Queue node uses: vcpu_running& vcpu_halted.
>> * Queue head uses: vcpu_running& vcpu_hashed.
>> */
>> @@ -202,6 +215,20 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
>> }
>>
>> /*
>> + * Return true if when it is time to check the previous node which is not
>> + * in a running state.
>> + */
>> +static inline bool
>> +pv_wait_early(struct pv_node *prev, int loop)
>> +{
>> +
>> + if ((loop& PV_PREV_CHECK_MASK) != 0)
>> + return false;
>> +
>> + return READ_ONCE(prev->state) != vcpu_running;
>> +}
> So it appears to me the sole purpose of PV_PREV_CHECK_MASK it to avoid
> touching the prev->state cacheline too hard. Yet that is not mentioned
> anywhere above.

Yes, that is true. I will add a comment to that effect.

>
>> +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.

for (;; waitcnt++) {
/*
+ * Set correct vCPU state to be used by queue node
wait-early
+ * mechanism.
+ */
+ WRITE_ONCE(pn->state, vcpu_running);
+
+ /*
* Set the pending bit in the active lock spinning loop to
* disable lock stealing. However, the pending bit check in
* pv_queued_spin_trylock_unfair() and the setting/clearing
@@ -374,6 +414,7 @@ static u32 pv_wait_head_lock(struct qspinlock *lock,
struct mcs_spinlock *node)
goto gotlock;
}
}
+ WRITE_ONCE(pn->state, vcpu_halted);

> FYI, I think I've now seen all patches ;-)

Thanks for the review. I will work on fixing the issues you identified
and issue a new patch series next week.

Cheers,
Longman

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