Re: [PATCH] generic dynamic per cpu refcounting

[Kent Overstreet <koverstreet@google.com>]

On Mon, Jan 28, 2013 at 01:36:22PM -0800, Tejun Heo wrote:
> On Mon, Jan 28, 2013 at 01:28:14PM -0800, Tejun Heo wrote:
> > But at that point, the operation is already global, so there gotta be
> > a lighter way to synchronize stuff than going through full grace
> > period.  ie. You can add a bias value before marking dead so that the
> > counter never reaches zero before all percpu counters are collected
> > and then unbias it right before putting the base ref, that way the
> > only way you can hit zero ref is all refs are actually zero.
> 
> Note that I'm saying that there's no need to distinguish between dying
> and dead.

Yeah I got that, I _think_ that makes me like the idea.

> The only thing percpu part should care about it whether
> percpu is on or off.  We need a full grace period to turn off percpu
> operations of any type but that should be the only case where full
> grace period is necessary.  The rest should be synchronizable via the
> usual global synchronization.  We probably can just test percpu
> variable against NULL and forget about the encoded state numbers.

Can't quite do that because initially, we use that variable for a
timestamp and when we're shutting down we don't want percpu_ref_get()
allocating percpu counters again.

You'll probably use that as an argument against dynamic allocation :P