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Message-ID: <d921273395b236bbc07bd7abd0359108b819448c.camel@linux.intel.com>
Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory
 hints
From:   Alexander Duyck <alexander.h.duyck@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     rkrcmar@redhat.com, x86@kernel.org, mingo@redhat.com, bp@alien8.de,
        hpa@zytor.com, pbonzini@redhat.com, tglx@linutronix.de,
        akpm@linux-foundation.org
Date:   Mon, 04 Feb 2019 12:42:22 -0800
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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.