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

From: Jens Axboe
Date: Fri Feb 08 2013 - 09:50:19 EST

On Fri, Feb 08 2013, 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@xxxxxxxxxx>
> 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'd feel a lot better deferring the whole aio/dio performance series for
one merge window. There's very little point in rushing it, and I don't
think it's been reviewed/tested enough yet.

Jens Axboe

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