linux-kernel - Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw

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Date:	Tue, 14 Jul 2015 14:17:04 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>
CC:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore

On 07/14/2015 06:37 AM, Oleg Nesterov wrote:
> On 07/14, Jan Kara wrote:
>> So unless
>> I'm missing something and there is a significant performance advantage to
>> Dave's patches I'm all for using a generic primitive you suggest.
> 
> I think percpu_rw_semaphore looks a bit better. And even a bit faster.
> And it will not block __sb_start_write() entirely while freeze_super()
> sleeps in synchronize_rcu().

That's true, but freeze_super() and the code blocked by it is a
super-rare path compared with write().

> freeze_super() should be faster too after rcu_sync changes, but this
> is not that important.
> 
> But again, to me the main advantage is that we can use the generic
> primitives and remove this nontrivial code in fs/super.c.
> 
>> Can you perhaps work with Dave on some common resolution?
> 
> Dave, what do you think? Will you agree with percpu_rw_semaphore ?

Using my little write-1-byte test (under will-it-scale), your 4 patches
improves the number of writes/sec by 12%.  My 3 patches improve the
number of writes/sec by 32%.

My patches manage to get rid of the memory barriers entirely in the fast
path.  Your approach keeps the barriers.

Test: https://www.sr71.net/~dave/intel/write1byte.c



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