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.l...@hpe.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
