From: Waiman Long <waiman.long@hpe.com>
To: Jason Low <jason.low2@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Ding Tianhong <dingtianhong@huawei.com>,
Jason Low <jason.low2@hpe.com>,
Davidlohr Bueso <dave@stgolabs.net>,
"Paul E. McKenney" <paulmck@us.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <Will.Deacon@arm.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()
Date: Mon, 15 Feb 2016 22:30:34 -0500 [thread overview]
Message-ID: <56C297DA.7060505@hpe.com> (raw)
In-Reply-To: <1455591654.2276.64.camel@j-VirtualBox>
On 02/15/2016 10:00 PM, Jason Low wrote:
> On Mon, 2016-02-15 at 18:55 -0500, Waiman Long wrote:
>> On 02/12/2016 03:40 PM, Peter Zijlstra wrote:
>>> On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
>>>> @@ -358,8 +373,8 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>>>> }
>>>>
>>>> mutex_set_owner(lock);
>>>> - osq_unlock(&lock->osq);
>>>> - return true;
>>>> + acquired = true;
>>>> + break;
>>>> }
>>>>
>>>> /*
>>>> @@ -380,7 +395,10 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>>>> cpu_relax_lowlatency();
>>>> }
>>>>
>>>> - osq_unlock(&lock->osq);
>>>> + if (!waiter)
>>>> + osq_unlock(&lock->osq);
>>>> + if (acquired || waiter)
>>>> + return acquired;
>>>> done:
>>>> /*
>>>> * If we fell out of the spin path because of need_resched(),
>>> Is there a reason to not also preempt in the wait-loop? Surely the same
>>> reason is still valid there too?
>> The waiter does check for need_sched(). So it will break out of the loop
>> and return false in this case. This causes the waiter to loop back and
>> goes to sleep if the lock can't be acquired. That is why I don't think
>> we need to do another schedule_preempt_disabled() here.
> The purpose of the additional reschedule point is to avoid delaying
> preemption, which still applies if the spinner is a waiter. If it is a
> waiter, the difference is that the delay isn't as long since it doesn't
> need to be added to the wait_list. Nonetheless, preemption delays can
> still occur, so I think the additional preemption point should also be
> there in the waiter case.
You are right. Taking the wait lock can introduce arbitrary delay. So I
will modify the patch to fall through and check for preemption.
Cheers,
Longman
next prev parent reply other threads:[~2016-02-16 3:30 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-12 17:32 [PATCH v2 0/4] locking/mutex: Enable optimistic spinning of lock waiter Waiman Long
2016-02-12 17:32 ` [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long
2016-02-12 20:23 ` Peter Zijlstra
2016-02-12 22:14 ` Davidlohr Bueso
2016-02-13 12:10 ` Peter Zijlstra
2016-02-13 18:14 ` Davidlohr Bueso
2016-02-16 2:15 ` Jason Low
2016-02-16 2:22 ` Jason Low
2016-02-16 8:53 ` Peter Zijlstra
2016-02-17 1:40 ` Waiman Long
2016-02-15 22:06 ` Waiman Long
2016-02-12 20:40 ` Peter Zijlstra
2016-02-15 23:55 ` Waiman Long
2016-02-16 3:00 ` Jason Low
2016-02-16 3:30 ` Waiman Long [this message]
2016-02-12 22:02 ` Davidlohr Bueso
2016-02-12 22:09 ` Davidlohr Bueso
2016-02-16 0:03 ` Waiman Long
2016-02-12 17:32 ` [PATCH v2 2/4] locking/mutex: Enable optimistic spinning of woken task in wait queue Waiman Long
2016-02-12 17:32 ` [PATCH v2 3/4] locking/mutex: Avoid missed wakeup of mutex waiter Waiman Long
2016-02-12 17:32 ` [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state Waiman Long
2016-02-12 20:18 ` Peter Zijlstra
2016-02-12 21:22 ` Waiman Long
2016-02-13 12:09 ` Peter Zijlstra
2016-02-16 8:51 ` [PATCH v2 0/4] locking/mutex: Enable optimistic spinning of lock waiter Peter Zijlstra
2016-02-17 1:39 ` Waiman Long
2016-03-22 3:19 ` Waiman Long
2016-03-22 9:59 ` Peter Zijlstra
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