linux-kernel - Re: aim7 performance regression by commit 5a50508 report from LKP

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Date:	Tue, 29 Jan 2013 17:06:20 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Huang Ying <ying.huang@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, lkp@...ux.intel.com
Subject: Re: aim7 performance regression by commit 5a50508 report from LKP

On Tue, Jan 29, 2013 at 09:44:00AM +0100, Ingo Molnar wrote:
> 
> * Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:
> 
> > [...]
> 
> Very nice measurements and analysis, thanks!
> 
> > As stated above, anybody can have a chance to own the lock in 
> > mutex once somebody release the lock. Well, there is only one 
> > to own the lock in rwsem write lock, and the one is known 
> > already: the one in the head of wait list. That would result 
> > to more contention in rwsem write lock case, especially if the 
> > one _will_ own the lock is not running now.
> 
> I think we should allow lock-steal between rwsem writers - that 
> will not hurt fairness as most rwsem fairness concerns relate to 
> reader vs. writer fairness.

Agreed, and I'm sure this will improve performance and may make this
performance regression go away. 

David, is that Ok to you? If so, I may have a try.

> 
> Am I correct to assume that all relevant users in this workload 
> are down_write() users?

Yes, as commit 5a50508 just convert all mutex to down_write.

Thanks.

	--yliu
> 
> You can see the type of lock use in:
> 
>    perf record -g
>    perf report
> 
> I bet that allowing rwsem writer lock-steal would improve other 
> workloads as well.
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