linux-kernel - Re: [PATCH v2 1/7] lib/dlock-list: Distributed and lock-protected lists

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Date:	Thu, 14 Jul 2016 10:51:09 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
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>,
	Christoph Lameter <cl@...ux-foundation.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 v2 1/7] lib/dlock-list: Distributed and lock-protected
 lists

Hello, Jan.

On Thu, Jul 14, 2016 at 04:35:47PM +0200, Jan Kara wrote:
> > > The current use case only need to use the regular lock functions. You are
> > > right that future use cases may require an irqsafe version of locks. I can
> > > either modify the code now to allow lock type selection at init time, for
> > > example, or defer it as a future enhancement when the need arises. What do
> > > you think?
> > 
> > The bulk of performance gain of dlist would come from being per-cpu
> > and I don't think it's likely that we'd see any noticeable difference
> > between irq and preempt safe operations.  Given that what's being
> > implemented is really low level operations, I'd suggest going with
> > irqsafe from the get-go.
> 
> I'm not sure here. i_sb_list for which percpu lists will be used is bashed
> pretty heavily under some workloads and the cost of additional interrupt
> disabling & enabling may be visible under those loads. Probably not in the
> cases where you get a boost from percpu lists but if the workload is mostly
> single-threaded, additional cpu cost may be measurable. So IMO we should
> check whether a load which creates tons of empty inodes in tmpfs from a
> single process doesn't regress with this change.

Sure, if it actually matters, we can always create separate preempt /
irq variants.

Thanks.

-- 
tejun