From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754881AbcGUWdM (ORCPT ); Thu, 21 Jul 2016 18:33:12 -0400 Received: from g2t4618.austin.hp.com ([15.73.212.83]:35280 "EHLO g2t4618.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754408AbcGUWdK (ORCPT ); Thu, 21 Jul 2016 18:33:10 -0400 Message-ID: <1469140171.2344.24.camel@j-VirtualBox> Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled From: Jason Low To: Waiman Long Cc: jason.low2@hpe.com, imre.deak@intel.com, Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , Chris Wilson , Daniel Vetter , Davidlohr Bueso , jason.low2@hp.com Date: Thu, 21 Jul 2016 15:29:31 -0700 In-Reply-To: <578FC4EE.1070000@hpe.com> References: <1468858607-20481-1-git-send-email-imre.deak@intel.com> <20160718171537.GC6862@twins.programming.kicks-ass.net> <1468864069.2367.21.camel@j-VirtualBox> <1468947205.31332.40.camel@intel.com> <1468969470.10247.15.camel@j-VirtualBox> <1468989556.10247.22.camel@j-VirtualBox> <578FC4EE.1070000@hpe.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > --- > > 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