linux-kernel - Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled

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Date:	Fri, 22 Jul 2016 11:44:54 -0700
From:	Jason Low <jason.low2@....com>
To:	imre.deak@...el.com
Cc:	jason.low2@....com, Waiman Long <waiman.long@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is
 disabled

On Fri, 2016-07-22 at 12:34 +0300, Imre Deak wrote:
> On to, 2016-07-21 at 15:29 -0700, Jason Low wrote:
> > On Wed, 2016-07-20 at 14:37 -0400, Waiman Long wrote:
> > > On 07/20/2016 12:39 AM, Jason Low wrote:
> > > > On Tue, 2016-07-19 at 16:04 -0700, Jason Low wrote:
> > > > > Hi Imre,
> > > > > 
> > > > > Here is a patch which prevents a thread from spending too much
> > > > > "time"
> > > > > waiting for a mutex in the !CONFIG_MUTEX_SPIN_ON_OWNER case.
> > > > > 
> > > > > Would you like to try this out and see if this addresses the
> > > > > mutex
> > > > > starvation issue you are seeing in your workload when
> > > > > optimistic
> > > > > spinning is disabled?
> > > > Although it looks like it didn't take care of the 'lock stealing'
> > > > case
> > > > in the slowpath. Here is the updated fixed version:
> > > > 
> > > > ---
> > > > Signed-off-by: Jason Low<jason.low2@....com>
> > > > ---
> > > >   include/linux/mutex.h  |  2 ++
> > > >   kernel/locking/mutex.c | 65
> > > > ++++++++++++++++++++++++++++++++++++++++++++------
> > > >   2 files changed, 60 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> > > > index 2cb7531..c1ca68d 100644
> > > > --- a/include/linux/mutex.h
> > > > +++ b/include/linux/mutex.h
> > > > @@ -57,6 +57,8 @@ struct mutex {
> > > >   #endif
> > > >   #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> > > >   	struct optimistic_spin_queue osq; /* Spinner MCS lock
> > > > */
> > > > +#else
> > > > +	bool yield_to_waiter; /* Prevent starvation when
> > > > spinning disabled */
> > > >   #endif
> > > >   #ifdef CONFIG_DEBUG_MUTEXES
> > > >   	void			*magic;
> > > 
> > > You don't need that on non-SMP system. So maybe you should put it
> > > under 
> > > #ifdef CONFIG_SMP block.
> > 
> > Right, maybe something like:
> > 
> >     #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> > 	...
> > 	...
> >     #elif !defined(CONFIG_SMP) /* If optimistic spinning disabled */
> >         bool yield_to_waiter;
> >     #endif
> > 
> > > > @@ -556,7 +595,8 @@ __mutex_lock_common(struct mutex *lock, long
> > > > state, unsigned int subclass,
> > > >   		 * other waiters. We only attempt the xchg if
> > > > the count is
> > > >   		 * non-negative in order to avoid unnecessary
> > > > xchg operations:
> > > >   		 */
> > > > -		if (atomic_read(&lock->count)>= 0&&
> > > > +		if ((!need_yield_to_waiter(lock) || loop>  1)&&
> > > > +		    atomic_read(&lock->count)>= 0&&
> > > >   		(atomic_xchg_acquire(&lock->count, -1) == 1))
> > > >   	
> > > 
> > > I think you need to reset the yield_to_waiter variable here when
> > > loop > 
> > > 1 instead of at the end of the loop.
> > 
> > So I think in the current state, only the top waiter would be able to
> > both set and clear the yield_to_waiter variable anyway. However, I
> > agree
> > that this detail is not obvious and it would be better to reset the
> > variable here when loop > 1 to make it more readable.
> 
> AFAICS an interruptible waiter behind the top waiter receiving a signal
> and grabbing the lock could also reset yield_to_waiter incorrectly in
> that way, increasing the top waiter's delay arbitrarily.

Okay, fair enough  :)

The reset will get moved so that only the waiter yielded to can call it.