linux-kernel - Re: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes

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Message-ID: <20111122134155.GA23599@localhost>
Date:	Tue, 22 Nov 2011 21:41:55 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: fix dirtied pages accounting on
 sub-page writes

On Tue, Nov 22, 2011 at 09:07:50PM +0800, Wu Fengguang wrote:
> On Tue, Nov 22, 2011 at 08:57:42PM +0800, Peter Zijlstra wrote:
> > On Tue, 2011-11-22 at 13:21 +0100, Jan Kara wrote:
> > > > +             __get_cpu_var(bdp_ratelimits)++;
> > >   I think you need preempt_disable() and preempt_enable() pair around
> > > __get_cpu_var(). Otherwise a process could get rescheduled in the middle of
> > > read-modify-write cycle... 
> > 
> > there's of course the this_cpu_inc(bdp_ratelimits); thing.
> > 
> > On x86 that'll turn into a single insn, on others it will add the
> > required preempt_disable/enable bits.
> 
> It's good to know that. But what if we don't really care which CPU
> data it's increasing, and can accept losing some increases due to the
> resulted race condition?

I just added a comment for it, hope it helps :)

                /*
                 * This is racy, however bdp_ratelimits merely serves as a
                 * gross safeguard. We don't really care the exact CPU it's
                 * charging to and the resulted inaccuracy is acceptable.
                 */
                __get_cpu_var(bdp_ratelimits)++;

Thanks,
Fengguang
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