linux-kernel - Re: performance delta after VFS i_mutex=>i

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Date:	Thu, 9 Jun 2016 12:25:52 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Davidlohr Bueso <dbueso@...e.de>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Jason Low <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: performance delta after VFS i_mutex=>i_rwsem conversion


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Waiman Long <waiman.long@....com> wrote:
> 
> > I do have a patchset that allow us to more accurately determine the state of
> > the lock owner.
> > 
> > locking/rwsem: Add reader-owned state to the owner field
> > http://www.spinics.net/lists/kernel/msg2258572.html
> > 
> > That should eliminate the performance gap between mutex and rwsem wrt
> > spinning when only writers are present. I am hoping that that patchset can
> > be queued for 4.8.
> 
> Yeah, so I actually had this series merged for testing last week, but a 
> complication with a prereq patch made me unmerge it. But I have no fundamental 
> objections, at all.
> 
> I also agree with Linus's general observation that we want to make 
> down_write()/up_write() match mutex performance characteristics.
> 
> I think kernel developers should fundamentally be able to switch between 
> mutex_lock()/unlock() and down_write()/up_write() and back, without noticing
> any high level behavioral changes.

Ok, these enhancements are now in the locking tree and are queued up for v4.8:

   git pull git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/core

Dave, you might want to check your numbers with these changes: is rwsem 
performance still significantly worse than mutex performance?

Thanks,

	Ingo