Messages in this thread | | | Date | Tue, 14 Jul 2015 21:24:13 -0400 | From | Waiman Long <> | Subject | Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time |
| |
On 07/13/2015 08:02 AM, Peter Zijlstra wrote: > On Sat, Jul 11, 2015 at 04:36:52PM -0400, Waiman Long wrote: >> @@ -181,9 +187,9 @@ static void pv_wait_node(struct mcs_spinlock *node) >> pv_wait(&pn->state, vcpu_halted); >> >> /* >> - * Reset the vCPU state to avoid unncessary CPU kicking >> + * Reset the state except when vcpu_hashed is set. >> */ >> - WRITE_ONCE(pn->state, vcpu_running); >> + cmpxchg(&pn->state, vcpu_halted, vcpu_running); > Why? Suppose we did get advanced into the hashed state, and then get a > (spurious) wakeup, this means we'll observe our ->locked == 1 condition > and fall out of pv_wait_node(). > > We'll then enter pv_wait_head(), which with your modification: > >> @@ -229,19 +244,42 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node) >> { >> struct pv_node *pn = (struct pv_node *)node; >> struct __qspinlock *l = (void *)lock; >> - struct qspinlock **lp = NULL; >> + struct qspinlock **lp; >> int loop; >> >> + /* >> + * Initialize lp to a non-NULL value if it has already been in the >> + * pv_hashed state so that pv_hash() won't be called again. >> + */ >> + lp = (READ_ONCE(pn->state) == vcpu_hashed) ? (struct qspinlock **)1 >> + : NULL; >> for (;;) { >> + WRITE_ONCE(pn->state, vcpu_running); > Will instantly and unconditionally write vcpu_running. > >
This code is kind of complicated. I am going to get rid of the current tri-state setup, and switch to a separate sync variable for defer kicking.
Cheers, Longman
| |