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

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Date:	Mon, 04 Aug 2014 11:44:19 -0400
From:	Waiman Long <waiman.long@...com>
To:	Jason Low <jason.low2@...com>
CC:	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>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic
 spinning

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.

-Longman
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