linux-kernel - Re: [PATCH v3 0/3] locking/mutex: Enable optimistic spinning of lock waiter

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Date:	Mon, 18 Jul 2016 16:50:24 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>, <jason.low2@...com>
Subject: Re: [PATCH v3 0/3] locking/mutex: Enable optimistic spinning of lock
 waiter

On 07/18/2016 01:59 PM, Jason Low wrote:
> On Tue, 2016-03-22 at 13:46 -0400, Waiman Long wrote:
>> v2->v3:
>>   - Remove patch 4 as it is not useful.
>>   - Allow need_resched() check for waiter&  add more comments about
>>     changes to address issues raised by PeterZ.
>>
>> v1->v2:
>>   - Set task state to running before doing optimistic spinning.
>>   - Add 2 more patches to handle possible missed wakeups and wasteful
>>     spinning in try_to_wake_up() function.
>>
>> This patchset is a variant of PeterZ's "locking/mutex: Avoid spinner
>> vs waiter starvation" patch. The major difference is that the
>> waiter-spinner won't enter into the OSQ used by the spinners. Instead,
>> it will spin directly on the lock in parallel with the queue head
>> of the OSQ. So there will be a bit more cacheline contention on the
>> lock cacheline, but that shouldn't cause noticeable impact on system
>> performance.
>>
>> This patchset tries to address 2 issues with Peter's patch:
>>
>>   1) Ding Tianhong still find that hanging task could happen in some cases.
>>   2) Jason Low found that there was performance regression for some AIM7
>>      workloads.
>>
>> By making the waiter-spinner to spin directly on the mutex, it will
>> increase the chance for the waiter-spinner to get the lock instead
>> of waiting in the OSQ for its turn.
>>
>> Patch 1 modifies the mutex_optimistic_spin() function to enable it
>> to be called by a waiter-spinner that doesn't need to go into the OSQ.
>>
>> Patch 2 modifies the mutex locking slowpath to make the waiter call
>> mutex_optimistic_spin() to do spinning after being waken up.
> Just in case, here was the Waiman's patch that I was referring to for
> addressing some of the starvation issues with mutex by allowing the
> waiter to do optimistic spinning after being woken up.
>
>> Patch 3 reverses the sequence of setting task state and changing
>> mutex count to -1 to prevent the possibility of missed wakeup.
>>
>> Patch 4 modifies the wakeup code to abandon the wakeup operation
>> while spinning on the on_cpu flag if the task has changed back to a
>> non-sleeping state.
>>
>> My own test on a 4-socket E7-4820 v3 system showed a regression of
>> about 4% in the high_systime workload with Peter's patch which this
>> new patch effectively eliminates.
>>
>> Testing on an 8-socket Westmere-EX server, however, has performance
>> change from -9% to than +140% on the fserver workload of AIM7
>> depending on how the system was set up.
>>
>> Waiman Long (3):
>>    locking/mutex: Add waiter parameter to mutex_optimistic_spin()
>>    locking/mutex: Enable optimistic spinning of woken task in wait queue
>>    locking/mutex: Avoid missed wakeup of mutex waiter
>>
>>   kernel/locking/mutex.c |  126 ++++++++++++++++++++++++++++++++++--------------
>>   1 files changed, 90 insertions(+), 36 deletions(-)

I have just sent out an updated patchset to address this issue.

Cheers,
Longman