linux-ext4 - Re: [PATCH 1/4] fs: Improve filesystem freezing handling

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Date:	Fri, 13 Jan 2012 11:12:20 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	Surbhi Palande <csurbhi@...il.com>,
	Kamal Mostafa <kamal@...onical.com>,
	Eric Sandeen <sandeen@...deen.net>,
	LKML <linux-kernel@...r.kernel.org>, xfs@....sgi.com,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <dchinner@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/4] fs: Improve filesystem freezing handling

On Fri 13-01-12 12:26:43, Dave Chinner wrote:
> On Thu, Jan 12, 2012 at 02:20:50AM +0100, Jan Kara wrote:
> > + *
> > + * Decrement number of writers to the filesystem and wake up possible
> > + * waiters wanting to freeze the filesystem.
> > + */
> > +void sb_end_write(struct super_block *sb)
> > +{
> > +#ifdef CONFIG_SMP
> > +	this_cpu_dec(sb->s_writers);
> > +#else
> > +	preempt_disable();
> > +	sb->s_writers--;
> > +	preempt_enable();
> > +#endif
> 
> I really dislike this type of open coded per-cpu counter
> implementation. I can't see that there is no good reason to use it
> over percpu_counters here which abstract all this mess away.
> 
> i.e. it is relatively rare that the per-cpu count will nest
> greater than the percpu_counter batch size (needs more than 32
> concurrent blocked active writes per CPU), so there is no
> significant overhead to using the percpu_counters here.
> 
> Indeed, if there are that many blocked writes per CPU, then the
> overhead of an occasional global counter update is going to be lost
> in the noise of everything else that is going on.
  Well, I just did it the way mnt_want_write / mnt_put_write does it. But
you are right that it's unnecessary so it's a good idea to switch the code
to using per-cpu counters. Thanks for the idea.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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