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

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Date:	Thu, 21 Jul 2016 15:29:31 -0700
From:	Jason Low <jason.low2@....com>
To:	Waiman Long <waiman.long@....com>
Cc:	jason.low2@....com, imre.deak@...el.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>, jason.low2@...com
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is
 disabled

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.

> > 		break;
> >
> > @@ -581,6 +621,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >   		spin_unlock_mutex(&lock->wait_lock, flags);
> >   		schedule_preempt_disabled();
> >   		spin_lock_mutex(&lock->wait_lock, flags);
> > +		do_yield_to_waiter(lock, loop);
> >   	}
> >   	__set_task_state(task, TASK_RUNNING);
> >
> > @@ -590,6 +631,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >   		atomic_set(&lock->count, 0);
> >   	debug_mutex_free_waiter(&waiter);
> >
> > +#ifndef CONFIG_MUTEX_SPIN_ON_OWNER
> > +	lock->yield_to_waiter = false;
> > +#endif
> > +
> 
> Maybe you should do the reset in an inline function instead.

Yes, this should be abstracted into a function like we do with
do_yield_to_waiter().


Jason