linux-kernel - Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

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Date:	Tue, 03 Feb 2015 09:16:52 -0800
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners


> > >  
> > > +	if (READ_ONCE(sem->owner))
> > > +		return true; /* new owner, continue spinning */
> > > +
> > 
> > Do you have some comparison data of whether it is more advantageous
> > to continue spinning when owner changes?  After the above change, 
> > rwsem will behave more like a spin lock for write lock and 
> > will keep spinning when the lock changes ownership.
> 
> But recall we still abort when need_resched, so the spinning isn't
> infinite. Never has been.
> 
> >  Now during heavy
> > lock contention, if we don't continue spinning and sleep, we may use the
> > clock cycles for actually running other threads. 
> 
> Under heavy contention, time spinning will force us to ultimately block
> anyway.

The question is under heavy contention, if we are going to block anyway,
won't it be more advantageous not to continue spinning so we can use
the cycles for useful task?  The original code assumes that if the lock
has switched owner, then we are under heavy contention and we can stop
spinning and block.  I think it'll be useful to have some
data comparing the two behaviors.

Thanks.

Tim


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