linux-kernel - Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

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Date:	Fri, 18 Feb 2011 22:33:46 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	suzuki@...ibm.com, vsrivatsa@...il.com
Subject: Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

> On Mon, Jan 24, 2011 at 01:56:53PM -0800, Jeremy Fitzhardinge wrote:

For some reason, I seem to be missing emails from your id/domain and hence had
missed this completely!

> >   * bits.  However, we need to be careful about this because someone
> >   * may just be entering as we leave, and enter the slowpath.
> >   */
> > -void __ticket_unlock_release_slowpath(struct arch_spinlock *lock)
> > +void __ticket_unlock_slowpath(struct arch_spinlock *lock)
> >  {
> >  	struct arch_spinlock old, new;
> >  
> >  	BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
> >  
> >  	old = ACCESS_ONCE(*lock);
> > -
> >  	new = old;
> > -	new.tickets.head += TICKET_LOCK_INC;
> >  
> >  	/* Clear the slowpath flag */
> >  	new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
> > +	if (new.tickets.head == new.tickets.tail)
> > +		cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
> >  
> > -	/*
> > -	 * If there's currently people waiting or someone snuck in
> > -	 * since we read the lock above, then do a normal unlock and
> > -	 * kick.  If we managed to unlock with no queued waiters, then
> > -	 * we can clear the slowpath flag.
> > -	 */
> > -	if (new.tickets.head != new.tickets.tail ||
> > -	    cmpxchg(&lock->head_tail,
> > -		    old.head_tail, new.head_tail) != old.head_tail) {
> > -		/* still people waiting */
> > -		__ticket_unlock_release(lock);
> > -	}
> > -
> > +	/* Wake up an appropriate waiter */
> >  	__ticket_unlock_kick(lock, new.tickets.head);
> 
> Does the __ticket_unlock_kick need to be unconditional?

I recall having tried optimizing it to be conditional, something along these
lines:

	if (new.ticket.head == new.tickets.tail) {
		cmpxchg();	
	} else {
		__ticket_unlock_kick(lock, new.tickets.head);
	}

but it didn't work for some reason. I left the call unconditional as was the
case previously based on that experiment.

- vatsa
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