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

[Alexander Duyck]

On Fri, 2019-02-08 at 16:05 -0500, Nitesh Narayan Lal wrote:
> On 2/7/19 1:44 PM, Alexander Duyck wrote:
> > On Thu, 2019-02-07 at 13:21 -0500, Luiz Capitulino wrote:
> > > On Mon, 04 Feb 2019 10:15:52 -0800
> > > Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote:
> > > 
> > > > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > > > 
> > > > 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@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  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
> > > > +
> > > >  #include <linux/range.h>
> > > >  extern struct range pfn_mapped[];
> > > >  extern int nr_pfn_mapped;
> > > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > > > index 5c93a65ee1e5..09c91641c36c 100644
> > > > --- a/arch/x86/kernel/kvm.c
> > > > +++ b/arch/x86/kernel/kvm.c
> > > > @@ -48,6 +48,7 @@
> > > >  #include <asm/tlb.h>
> > > >  
> > > >  static int kvmapf = 1;
> > > > +DEFINE_STATIC_KEY_FALSE(pv_free_page_hint_enabled);
> > > >  
> > > >  static int __init parse_no_kvmapf(char *arg)
> > > >  {
> > > > @@ -648,6 +649,15 @@ static void __init kvm_guest_init(void)
> > > >  	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
> > > >  		apic_set_eoi_write(kvm_guest_apic_eoi_write);
> > > >  
> > > > +	/*
> > > > +	 * The free page hinting doesn't add much value if page poisoning
> > > > +	 * is enabled. So we only enable the feature if page poisoning is
> > > > +	 * no present.
> > > > +	 */
> > > > +	if (!page_poisoning_enabled() &&
> > > > +	    kvm_para_has_feature(KVM_FEATURE_PV_UNUSED_PAGE_HINT))
> > > > +		static_branch_enable(&pv_free_page_hint_enabled);
> > > > +
> > > >  #ifdef CONFIG_SMP
> > > >  	smp_ops.smp_prepare_cpus = kvm_smp_prepare_cpus;
> > > >  	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
> > > > @@ -762,6 +772,19 @@ static __init int kvm_setup_pv_tlb_flush(void)
> > > >  }
> > > >  arch_initcall(kvm_setup_pv_tlb_flush);
> > > >  
> > > > +void __arch_free_page(struct page *page, unsigned int order)
> > > > +{
> > > > +	/*
> > > > +	 * Limit hints to blocks no smaller than pageblock in
> > > > +	 * size to limit the cost for the hypercalls.
> > > > +	 */
> > > > +	if (order < KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER)
> > > > +		return;
> > > > +
> > > > +	kvm_hypercall2(KVM_HC_UNUSED_PAGE_HINT, page_to_phys(page),
> > > > +		       PAGE_SIZE << order);
> > > 
> > > Does this mean that the vCPU executing this will get stuck
> > > here for the duration of the hypercall? Isn't that too long,
> > > considering that the zone lock is taken and madvise in the
> > > host block on semaphores?
> > 
> > I'm pretty sure the zone lock isn't held when this is called. The lock
> > isn't acquired until later in the path. This gets executed just before
> > the page poisoning call which would take time as well since it would
> > have to memset an entire page. This function is called as a part of
> > free_pages_prepare, the zone locks aren't acquired until we are calling
> > into either free_one_page and a few spots before calling
> > __free_one_page.
> > 
> > My other function in patch 4 which does this from inside of
> > __free_one_page does have to release the zone lock since it is taken
> > there.
> > 
> 
> Considering hypercall's are costly, will it not make sense to coalesce
> the pages you are reporting and make a single hypercall for a bunch of
> pages?

That is what I am doing with this code and patch 4. I am only making
the call when I have been given a page that is 2M or larger. As such I
am only making one hypercall for every 512 4K pages.

So for example on my test VMs with 8G of RAM I see only about 3K calls
when it ends up freeing all of the application memory which is about 6G
after my test has ended.