linux-kernel - Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

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Date:	Thu, 21 Jan 2016 18:41:08 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ding Tianhong <dingtianhong@...wei.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Jason Low <jason.low2@...com>, Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list
 is not NULL.

On Thu, Jan 21, 2016 at 01:23:09PM -0800, Tim Chen wrote:
> On Thu, 2016-01-21 at 17:29 +0800, Ding Tianhong wrote:
> 
> > 
> > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > index 0551c21..596b341 100644
> > --- a/kernel/locking/mutex.c
> > +++ b/kernel/locking/mutex.c
> > @@ -256,7 +256,7 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
> >  	struct task_struct *owner;
> >  	int retval = 1;
> >  
> > -	if (need_resched())
> > +	if (need_resched() || atomic_read(&lock->count) == -1)
> >  		return 0;
> >  
> 
> One concern I have is this change will eliminate any optimistic spinning
> as long as there is a waiter.  Is there a middle ground that we
> can allow only one spinner if there are waiters?  
> 
> In other words, we allow spinning when
> atomic_read(&lock->count) == -1 but there is no one on the
> osq lock that queue up the spinners (i.e. no other process doing
> optimistic spinning).
> 
> This could allow a bit of spinning without starving out the waiters.

I did some testing, which exposed it to the 0day test robot, which
did note some performance differences.  I was hoping that it would
clear up some instability from other patches, but no such luck.  ;-)

							Thanx, Paul