linux-kernel - Re: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic spinning

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Date:	Wed, 13 Aug 2014 12:41:06 -0400
From:	Waiman Long <waiman.long@...com>
To:	Dave Chinner <david@...morbit.com>
CC:	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <davidlohr@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic
 spinning

On 08/13/2014 01:51 AM, Dave Chinner wrote:
> On Mon, Aug 04, 2014 at 11:44:19AM -0400, Waiman Long wrote:
>> On 08/04/2014 12:10 AM, Jason Low wrote:
>>> On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote:
>>>> The rwsem_can_spin_on_owner() function currently allows optimistic
>>>> spinning only if the owner field is defined and is running. That is
>>>> too conservative as it will cause some tasks to miss the opportunity
>>>> of doing spinning in case the owner hasn't been able to set the owner
>>>> field in time or the lock has just become available.
>>>>
>>>> This patch enables more aggressive use of optimistic spinning by
>>>> assuming that the lock is spinnable unless proved otherwise.
>>>>
>>>> Signed-off-by: Waiman Long<Waiman.Long@...com>
>>>> ---
>>>>   kernel/locking/rwsem-xadd.c |    2 +-
>>>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
>>>> index d058946..dce22b8 100644
>>>> --- a/kernel/locking/rwsem-xadd.c
>>>> +++ b/kernel/locking/rwsem-xadd.c
>>>> @@ -285,7 +285,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
>>>>   static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>>>>   {
>>>>   	struct task_struct *owner;
>>>> -	bool on_cpu = false;
>>>> +	bool on_cpu = true;	/* Assume spinnable unless proved not to be */
>>> Hi,
>>>
>>> So "on_cpu = true" was recently converted to "on_cpu = false" in order
>>> to address issues such as a 5x performance regression in the xfs_repair
>>> workload that was caused by the original rwsem optimistic spinning code.
>>>
>>> However, patch 4 in this patchset does address some of the problems with
>>> spinning when there are readers. CC'ing Dave Chinner, who did the
>>> testing with the xfs_repair workload.
>>>
>> This patch set enables proper reader spinning and so the problem
>> that we see with xfs_repair workload should go away. I should have
>> this patch after patch 4 to make it less confusing. BTW, patch 3 can
>> significantly reduce spinlock contention in rwsem. So I believe the
>> xfs_repair workload should run faster with this patch than both 3.15
>> and 3.16.
> I see lots of handwaving. I documented the test I ran when I
> reported the problem so anyone with a 16p system and an SSD can
> reproduce it. I don't have the bandwidth to keep track of the lunacy
> of making locks scale these days - that's what you guys are doing.
>
> I gave you a simple, reliable workload that is extremely sensitive
> to rwsem perturbations, so you should be adding it to your
> regression tests rather than leaving it for others to notice you
> screwed up....
>
> Cheers,
>
> Dave.

If you can send me a rwsem workload that I can use for testing purpose, 
it will be highly appreciated.

-Longman
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