Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

Alexander Duyck
2019-02-04T20:42:25Z

On Mon, 2019-02-04 at 11:44 -0800, Dave Hansen wrote:
> On 2/4/19 10:15 AM, Alexander Duyck wrote:
> > +#ifdef CONFIG_KVM_GUEST
> > +#include <linux/jump_label.h>
> > +extern struct static_key_false pv_free_page_hint_enabled;
> > +
> > +#define HAVE_ARCH_FREE_PAGE
> > +void __arch_free_page(struct page *page, unsigned int order);
> > +static inline void arch_free_page(struct page *page, unsigned int order)
> > +{
> > + if (static_branch_unlikely(&pv_free_page_hint_enabled))
> > + __arch_free_page(page, order);
> > +}
> > +#endif
>
> So, this ends up with at least a call, a branch and a ret added to the
> order-0 paths, including freeing pages to the per-cpu-pageset lists.
> That seems worrisome.
>
> What performance testing has been performed to look into the overhead
> added to those paths?

So far I haven't done much in the way of actual performance testing.
Most of my tests have been focused on "is this doing what I think it is
supposed to be doing".

I have been debating if I want to just move the order checks to include
them in the inline functions. In that case we would end up essentially
just jumping over the call code.