diff options
Diffstat (limited to 'kernel/locking/qspinlock_paravirt.h')
-rw-r--r-- | kernel/locking/qspinlock_paravirt.h | 46 |
1 files changed, 44 insertions, 2 deletions
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h index ace60a4..87bb235 100644 --- a/kernel/locking/qspinlock_paravirt.h +++ b/kernel/locking/qspinlock_paravirt.h @@ -23,6 +23,20 @@ #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. This is to ensure that we won't + * pound on the cacheline of the previous node too heavily. + */ +#define PV_PREV_CHECK_MASK 0xff + +/* * Queue node uses: vcpu_running & vcpu_halted. * Queue head uses: vcpu_running & vcpu_hashed. */ @@ -235,6 +249,20 @@ 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; +} + +/* * Initialize the PV part of the mcs_spinlock node. */ static void pv_init_node(struct mcs_spinlock *node) @@ -252,17 +280,23 @@ static void pv_init_node(struct mcs_spinlock *node) * pv_kick_node() is used to set _Q_SLOW_VAL and fill in hash table on its * behalf. */ -static void pv_wait_node(struct mcs_spinlock *node) +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(); } @@ -280,6 +314,7 @@ static void pv_wait_node(struct mcs_spinlock *node) if (!READ_ONCE(node->locked)) { qstat_inc(qstat_pv_wait_node, true); qstat_inc(qstat_pv_wait_again, waitcnt); + qstat_inc(qstat_pv_wait_early, wait_early); pv_wait(&pn->state, vcpu_halted); } @@ -365,6 +400,12 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node) 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 before attempting to acquire the lock. */ @@ -402,6 +443,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node) goto gotlock; } } + WRITE_ONCE(pn->state, vcpu_halted); qstat_inc(qstat_pv_wait_head, true); qstat_inc(qstat_pv_wait_again, waitcnt); pv_wait(&l->locked, _Q_SLOW_VAL); |