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

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Message-ID: <20150715181920.GA1797@redhat.com>
Date:	Wed, 15 Jul 2015 20:19:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Hansen <dave.hansen@...ux.intel.com>,
	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 07/15, Jan Kara wrote:
>
> On Tue 14-07-15 14:41:13, Dave Hansen wrote:
> >
> > I looked at it again.  I tested with this patch in addition to the ones
> > modifying __sb_start/end_write():
> >
> > 	https://lkml.org/lkml/2015/6/24/682
> >
> > That is where the performance delta came from.  Your patches (plus the
> > fsnotify optimization) perform very similarly to my approach.
> >
> > Yours remove so much code that I think they are the preferable approach.
> >
> > They don't compile with lockdep on, btw. :)
>
> Great, thanks for hashing it out.

Yes, thanks Dave!

> So I'm also in favor of Oleg's approach
> as well. We just have to wait until he fixes the outstanding issues with
> his code.

Yes. I'll try to make the working version, hopefully this week.

But,

> Dave, just send your fsnotify patch separately to AKPM - he
> usually merges fsnotify stuff.

Perhaps it makes to merge other 2 patches from Dave first? (those which
change __sb_start/end_write to rely on RCU). Afaics these changes are
straightforward and correct. Although I'd suggest to use preempt_disable()
and synchronize_sched() instead. I will be happy to (try to) make this
conversion on top of his changes.

Because I do not want to delay the performance improvements and I do not
know when exactly I'll send the next version: I need to finish the previous
discussion about rcu_sync first. And the necessary changes in fs/super.c
depend on whether percpu_rw_semaphore will have rcu_sync or not (not too
much, only destroy_super() depends, but still).

And of course, I am worried that I missed something and percpu_rw_semaphore
can't work for some reason. The code in fs/super.c looks simple, but it
seems that filesystems do the "strange" things with lockdep at least.

Oleg.

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