On Fri, 15 Dec 2017 11:04:43 +0100 Michal Hocko <[email protected]> wrote:
> On Thu 14-12-17 12:42:46, Andrew Morton wrote: > > On Thu, 14 Dec 2017 16:17:18 +0100 Michal Hocko <[email protected]> wrote: > > > > > > as fast as possible, SRCU instead of reference count is used to > > > > implement get/put_swap_device(). From get_swap_device() to > > > > put_swap_device(), the reader side of SRCU is locked, so > > > > synchronize_srcu() in swapoff() will wait until put_swap_device() is > > > > called. > > > > > > It is quite unfortunate to pull SRCU as a dependency to the core kernel. > > > Different attempts to do this have failed in the past. This one is > > > slightly different though because I would suspect that those tiny > > > systems do not configure swap. But who knows, maybe they do. > > > > > > Anyway, if you are worried about performance then I would expect some > > > numbers to back that worry. So why don't simply start with simpler > > > ref count based and then optimize it later based on some actual numbers. > > > Btw. have you considered pcp refcount framework. I would suspect that > > > this would give you close to SRCU performance. > > > > <squeaky-wheel>Or use stop_kernel() ;)</squeaky-wheel> > > well, stop_kernel is a _huge_ hammer. But it's very simple and requires zero code changes on the fast path. This makes it appropriate for swapoff! > I think we can do much better > without a large complexity. A simple ref counting (or pcp refcounting if > the former has measurable complexity) should do just fine. I'd like to be able to compare the implementations ;)
