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Message-ID: <55A57C50.1080406@linux.intel.com>
Date: Tue, 14 Jul 2015 14:17:04 -0700
From: Dave Hansen <dave.hansen@linux.intel.com>
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To: Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>
CC: Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Davidlohr Bueso <dave@stgolabs.net>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore
References: <20150713212536.GA13855@redhat.com> <20150714104810.GB24369@quack.suse.cz> <20150714133731.GA24837@redhat.com>
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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