linux-kernel - Re: [PATCH 23/32] Generic dynamic per cpu refcounting

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Date:	Fri, 8 Feb 2013 06:44:08 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
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,
	tytso@....edu, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting

(cc'ing Andrew)

On Wed, Dec 26, 2012 at 06:00:02PM -0800, Kent Overstreet wrote:
> 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.
> 
> It also implements two stage shutdown, as we need it to tear down the
> percpu counts. Before dropping the initial refcount, you must call
> percpu_ref_kill(); this puts the refcount in "shutting down mode" and
> switches back to a single atomic refcount with the appropriate barriers
> (synchronize_rcu()).
> 
> It's also legal to call percpu_ref_kill() multiple times - it only
> returns true once, so callers don't have to reimplement shutdown
> synchronization.
> 
> For the sake of simplicity/efficiency, the heuristic is pretty simple -
> it just switches to percpu refcounting if there are more than x gets
> in one second (completely arbitrarily, 4096).
> 
> It'd be more correct to count the number of cache misses or something
> else more profile driven, but doing so would require accessing the
> shared ref twice per get - by just counting the number of gets(), we can
> stick that counter in the high bits of the refcount and increment both
> with a single atomic64_add(). But I expect this'll be good enough in
> practice.
> 
> Signed-off-by: Kent Overstreet <koverstreet@...gle.com>

What's the status of this series?  The percpu-refcnt part is still
going through review and the merge window is opening up pretty soon.
Kent, Andrew?

Thanks.

-- 
tejun
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