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

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Message-ID: <20130208211735.GG27179@google.com>
Date:	Fri, 8 Feb 2013 13:17:35 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.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,
	tytso@....edu, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting

On Fri, Feb 08, 2013 at 06:44:08AM -0800, Tejun Heo wrote:
> (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?

I don't think the percpu-refcount stuff should be blocking, the existing
code in Andrew's tree isn't ideal w.r.t. the api but it is stable and
tested.

I have patches on top of that that incorporate most of your ideas and
are definite improvements, I've just been holding off on them because I
want to come up with a good torture test (mainly because of the bias
mechanism for teardown).
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