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

> 
> * Waiman Long <waiman.long@hpe.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