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.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.
>
> 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.
