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

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Date:   Mon, 04 Feb 2019 12:42:22 -0800
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, x86@...nel.org, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, pbonzini@...hat.com, tglx@...utronix.de,
        akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory
 hints

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.