linux-kernel - Re: [PATCH v4 0/5] vfs: Use dlock list for SB's s

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Date:	Mon, 25 Jul 2016 13:41:32 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Waiman Long <Waiman.Long@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v4 0/5] vfs: Use dlock list for SB's s_inodes list

Hello, Christoph.

On Mon, Jul 25, 2016 at 08:48:25AM -0500, Christoph Lameter wrote:
> On Fri, 22 Jul 2016, Waiman Long wrote:
> 
> >  - Add a new patch to make the percpu head structure cacheline aligned
> >    to prevent cacheline contention from disrupting the performance
> >    of nearby percpu variables.
> 
> It would be better not to use the percpu allocation etc for this.
> Given the frequency of off node data access I would say that the data
> structure does not qualify as per cpu data. You have per cpu data items
> yes but this is not used as per cpu data.

I don't get it.  What's the harm of using percpu memory here?  Other
percpu data structures have remote access too.  They're to a lower
degree but I don't see a clear demarcation line and making addtions
per-cpu seems to have significant benefits here.  If there's a better
way of splitting the list and locking, sure, let's try that but short
of that I don't see anything wrong with doing this per-cpu.

Thanks.

-- 
tejun