Re: [PATCH] locking/osq: Drop the overload of osq lock

[Boqun Feng <boqun.feng@gmail.com>]

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On Sun, Jun 26, 2016 at 03:08:20PM +0800, panxinhui wrote:
[snip]
> > @@ -106,6 +109,9 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> > 	node->prev = prev;
> > 	WRITE_ONCE(prev->next, node);
> > 
> > +	old = old - 1;
> > +	vpc = vcpu_preempt_count();
> > +
> > 	/*
> > 	 * Normally @prev is untouchable after the above store; because at that
> > 	 * moment unlock can proceed and wipe the node element from stack.
> > @@ -118,8 +124,14 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> > 	while (!READ_ONCE(node->locked)) {
> > 		/*
> > 		 * If we need to reschedule bail... so we can block.
> > +		 * An over-committed guest with more vCPUs than pCPUs
> > +		 * might fall in this loop and cause a huge overload.
> > +		 * This is because vCPU A(prev) hold the osq lock and yield out,
> > +		 * vCPU B(node) wait ->locked to be set, IOW, wait till
> > +		 * vCPU A run and unlock the osq lock.
> > +		 * NOTE that vCPU A and vCPU B might run on same physical cpu.
> > 		 */
> > -		if (need_resched())
> > +		if (need_resched() || vcpu_is_preempted(old) || vcpu_has_preempted(vpc))
> > 			goto unqueue;
> > 
> 
> the prev might change, so we need  read node->prev every loop, then check vcpu preempted.
> 

Right you are on the possibility of the prev's change, however, even if
we reread node->prev, the prev is still not stable after we read, that
is the prev can change after we read in the loop and before we check the
vcpu preemption in the next loop, therefore whether the reread is
worthwhile, depends on some real tests I think.

Regards,
Boqun

> > 		cpu_relax_lowlatency();
> 

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