Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1182771

Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

Show all headers | View raw

On Mon, Jul 13, 2015 at 02:02:02PM +0200, 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;

Because that ^

> >  	for (;;) {
> > +		WRITE_ONCE(pn->state, vcpu_running);
> 
> Will instantly and unconditionally write vcpu_running.
> 
> 
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time Peter Zijlstra <peterz@infradead.org> - 2015-07-13 14:10 +0200
  Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time Peter Zijlstra <peterz@infradead.org> - 2015-07-13 14:40 +0200
  Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time Waiman Long <waiman.long@hp.com> - 2015-07-15 03:30 +0200

csiph-web