Messages in this thread | | | Date | Wed, 15 Jul 2015 22:18:35 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v2 5/6] locking/pvqspinlock: Opportunistically defer kicking to unlock time |
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On 07/15/2015 06:03 AM, Peter Zijlstra wrote: > On Tue, Jul 14, 2015 at 10:13:36PM -0400, Waiman Long wrote: >> +static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node) >> { >> struct pv_node *pn = (struct pv_node *)node; >> >> + if (xchg(&pn->state, vcpu_running) == vcpu_running) >> + return; >> + >> /* >> + * Kicking the next node at lock time can actually be a bit faster >> + * than doing it at unlock time because the critical section time >> + * overlaps with the wakeup latency of the next node. However, if the >> + * VM is too overcommmitted, it can happen that we need to kick the >> + * CPU again at unlock time (double-kick). To avoid that and also to >> + * fully utilize the kick-ahead functionality at unlock time, >> + * the kicking will be deferred under either one of the following >> + * 2 conditions: >> * >> + * 1) The VM guest has too few vCPUs that kick-ahead is not even >> + * enabled. In this case, the chance of double-kick will be >> + * higher. >> + * 2) The node after the next one is also in the halted state. >> * >> + * In this case, the hashed flag is set to indicate that hashed >> + * table has been filled and _Q_SLOW_VAL is set. >> */ >> - if (xchg(&pn->state, vcpu_running) == vcpu_halted) { >> - pvstat_inc(pvstat_lock_kick); >> - pv_kick(pn->cpu); >> + if ((!pv_kick_ahead || pv_get_kick_node(pn, 1))&& >> + (xchg(&pn->hashed, 1) == 0)) { >> + struct __qspinlock *l = (void *)lock; >> + >> + /* >> + * As this is the same vCPU that will check the _Q_SLOW_VAL >> + * value and the hash table later on at unlock time, no atomic >> + * instruction is needed. >> + */ >> + WRITE_ONCE(l->locked, _Q_SLOW_VAL); >> + (void)pv_hash(lock, pn); >> + return; >> } >> + >> + /* >> + * Kicking the vCPU even if it is not really halted is safe. >> + */ >> + pvstat_inc(pvstat_lock_kick); >> + pv_kick(pn->cpu); >> } >> >> /* >> @@ -513,6 +545,13 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node) >> cpu_relax(); >> } >> >> + if (!lp&& (xchg(&pn->hashed, 1) == 1)) >> + /* >> + * The hashed table& _Q_SLOW_VAL had been filled >> + * by the lock holder. >> + */ >> + lp = (struct qspinlock **)-1; >> + >> if (!lp) { /* ONCE */ >> lp = pv_hash(lock, pn); >> /* > *groan*, so you complained the previous version of this patch was too > complex, but let me say I vastly preferred it to this one :/
I said it was complex as maintaining a tri-state variable needed more thought than 2 bi-state variables. I can revert it back to the tri-state variable as doing an unconditional kick in unlock simplifies the code at pv_wait_head().
Cheers, Longman
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