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

[Alexander Duyck <alexander.duyck@gmail.com>]

On Mon, Feb 4, 2019 at 4:03 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>
> > On Feb 4, 2019, at 3:37 PM, Alexander Duyck <alexander.h.duyck@linux.intel.com> wrote:
> >
> > On Mon, 2019-02-04 at 15:00 -0800, Nadav Amit wrote:
> >>> On Feb 4, 2019, at 10:15 AM, Alexander Duyck <alexander.duyck@gmail.com> wrote:
> >>>
> >>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>>
> >>> Add guest support for providing free memory hints to the KVM hypervisor for
> >>> freed pages huge TLB size or larger. I am restricting the size to
> >>> huge TLB order and larger because the hypercalls are too expensive to be
> >>> performing one per 4K page. Using the huge TLB order became the obvious
> >>> choice for the order to use as it allows us to avoid fragmentation of higher
> >>> order memory on the host.
> >>>
> >>> I have limited the functionality so that it doesn't work when page
> >>> poisoning is enabled. I did this because a write to the page after doing an
> >>> MADV_DONTNEED would effectively negate the hint, so it would be wasting
> >>> cycles to do so.
> >>>
> >>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>> ---
> >>> arch/x86/include/asm/page.h |   13 +++++++++++++
> >>> arch/x86/kernel/kvm.c       |   23 +++++++++++++++++++++++
> >>> 2 files changed, 36 insertions(+)
> >>>
> >>> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
> >>> index 7555b48803a8..4487ad7a3385 100644
> >>> --- a/arch/x86/include/asm/page.h
> >>> +++ b/arch/x86/include/asm/page.h
> >>> @@ -18,6 +18,19 @@
> >>>
> >>> struct page;
> >>>
> >>> +#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
> >>
> >> This patch and the following one assume that only KVM should be able to hook
> >> to these events. I do not think it is appropriate for __arch_free_page() to
> >> effectively mean “kvm_guest_free_page()”.
> >>
> >> Is it possible to use the paravirt infrastructure for this feature,
> >> similarly to other PV features? It is not the best infrastructure, but at least
> >> it is hypervisor-neutral.
> >
> > I could probably tie this into the paravirt infrastructure, but if I
> > did so I would probably want to pull the checks for the page order out
> > of the KVM specific bits and make it something we handle in the inline.
> > Doing that I would probably make it a paravirtual hint that only
> > operates at the PMD level. That way we wouldn't incur the cost of the
> > paravirt infrastructure at the per 4K page level.
>
> If I understand you correctly, you “complain” that this would affect
> performance.

It wasn't so much a "complaint" as an "observation". What I was
getting at is that if I am going to make it a PV operation I might set
a hard limit on it so that it will specifically only apply to huge
pages and larger. By doing that I can justify performing the screening
based on page order in the inline path and avoid any PV infrastructure
overhead unless I have to incur it.

> While it might be, you may want to check whether the already available
> tools can solve the problem:
>
> 1. You can use a combination of static-key and pv-ops - see for example
> steal_account_process_time()

Okay, I was kind of already heading in this direction. The static key
I am using now would probably stay put.

> 2. You can use callee-saved pv-ops.
>
> The latter might anyhow be necessary since, IIUC, you change a very hot
> path. So you may want have a look on the assembly code of free_pcp_prepare()
> (or at least its code-size) before and after your changes. If they are too
> big, a callee-saved function might be necessary.

I'll have to take a look. I will spend the next couple days
familiarizing myself with the pv-ops infrastructure.