linux-kernel - Re: [PATCH 22/25] Generic dynamic per cpu refcounting

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Message-ID: <20121129185720.GE15094@google.com>
Date:	Thu, 29 Nov 2012 10:57:20 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, zab@...hat.com, bcrl@...ck.org,
	jmoyer@...hat.com, axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting

On Thu, Nov 29, 2012 at 10:45:04AM -0800, Andi Kleen wrote:
> Kent Overstreet <koverstreet@...gle.com> writes:
> 
> > This implements a refcount with similar semantics to
> > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t
> > but dynamically switches to per cpu refcounting when the rate of
> > gets/puts becomes too high.
> 
> This will only work if you put on the same CPU as you get, right?

Nope, no such restriction.

> In this case I would rather use RCU. It's clearly unusable for anything 
> blocking (or not get_cpu) Normally RCU already handles the "ref count for short non
> blocking case"

The kioctx refcount isn't held for short nonblocking duration,
io_getevents() holds it and may block for arbitrarily long. Maybe SRCU
could be made to work for it (I havent' really looked at the RCU
variants) but it doesn't seem like a good idea.

This thing really is just a refcount, the percpu part isn't exposed to
the user at all.
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