Messages in this thread | | | Date | Fri, 6 Nov 2015 16:01:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning |
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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.
> +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?
FYI, I think I've now seen all patches ;-)
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