linux-kernel - Re: [PATCH] generic dynamic per cpu refcounting

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Message-ID: <20130128185552.GD22465@mtj.dyndns.org>
Date:	Mon, 28 Jan 2013 10:55:52 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, srivatsa.bhat@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] generic dynamic per cpu refcounting

Hey, Kent.

On Mon, Jan 28, 2013 at 10:49:33AM -0800, Kent Overstreet wrote:
> Yeah. It'd be really nice if it was doable without synchronize_rcu(),
> but it'd definitely make get/put heavier.
> 
> Though, re. close() - considering we only need a synchronize_rcu() if
> the ref was in percpu mode, I wonder if that would be a dealbreaker. I
> have no clue myself.

The problem is that the performance drop (or latency increase) in
patheological cases would be catastrophic.  We're talking about
possibly quite a few millisecs of delay between each close().  When
done sequentially for large number of files, it gets ugly.  It becomes
a dangerous optimization to make.

> Getting rid of synchronize_rcu would basically require turning get and
> put into cmpxchg() loops - even in the percpu fastpath. However, percpu
> mode would still be getting rid of the shared cacheline contention, we'd
> just be adding another branch that can be safely marked unlikely() - and
> my current version has one of those already, so two branches instead of
> one in the fast path.

Or offer an asynchrnous interface so that high-frequency users don't
end up inserting synchronize_sched() between each call.  It makes the
interface more complex and further away from simple atomic_t
replacement tho.

Thanks.

-- 
tejun
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