From: Waiman Long <waiman.long@hpe.com>
To: Jason Low <jason.low2@hpe.com>
Cc: <imre.deak@intel.com>, Peter Zijlstra <peterz@infradead.org>,
<linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
Chris Wilson <chris@chris-wilson.co.uk>,
Daniel Vetter <daniel.vetter@intel.com>,
Davidlohr Bueso <dave@stgolabs.net>, <jason.low2@hp.com>
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled
Date: Fri, 22 Jul 2016 14:01:18 -0400 [thread overview]
Message-ID: <57925F6E.5030802@hpe.com> (raw)
In-Reply-To: <1469140171.2344.24.camel@j-VirtualBox>
On 07/21/2016 06:29 PM, 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@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.
You should only reset the variable when loop > 1. You may also need to
check in the error exit path as well.
Cheers,
Longman
prev parent reply other threads:[~2016-07-22 18:01 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-18 16:16 [RFC] locking/mutex: Fix starvation of sleeping waiters Imre Deak
2016-07-18 17:15 ` Peter Zijlstra
2016-07-18 17:47 ` Jason Low
2016-07-19 16:53 ` Imre Deak
2016-07-19 22:57 ` Jason Low
2016-07-19 23:04 ` [RFC] Avoid mutex starvation when optimistic spinning is disabled Jason Low
2016-07-20 4:39 ` Jason Low
2016-07-20 13:29 ` Imre Deak
2016-07-21 20:57 ` Jason Low
2016-07-22 17:55 ` Waiman Long
2016-07-22 18:03 ` Davidlohr Bueso
2016-07-22 18:29 ` Imre Deak
2016-07-22 19:26 ` Davidlohr Bueso
2016-07-22 19:53 ` Imre Deak
2016-07-20 18:37 ` Waiman Long
2016-07-21 22:29 ` Jason Low
2016-07-22 9:34 ` Imre Deak
2016-07-22 18:44 ` Jason Low
2016-07-22 18:01 ` Waiman Long [this message]
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