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From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
To: Nitesh Narayan Lal <nitesh@redhat.com>,
	Luiz Capitulino <lcapitulino@redhat.com>,
	Alexander Duyck <alexander.duyck@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, 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
Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints
Date: Fri, 08 Feb 2019 13:31:36 -0800
Message-ID: <e6c9ec462b50f2d6a33416e16b42d995236e447e.camel@linux.intel.com> (raw)
In-Reply-To: <5f5c03ac-0d21-d92b-1772-f26773437019@redhat.com>

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@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
> > > > +
> > > >  #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.


  reply index

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-04 18:15 [RFC PATCH 0/4] kvm: Report unused guest pages to host Alexander Duyck
2019-02-04 18:15 ` [RFC PATCH 1/4] madvise: Expose ability to set dontneed from kernel Alexander Duyck
2019-02-04 18:15 ` [RFC PATCH 2/4] kvm: Add host side support for free memory hints Alexander Duyck
2019-02-10  0:44   ` Michael S. Tsirkin
2019-02-11 17:34     ` Alexander Duyck
2019-02-11 17:36       ` Michael S. Tsirkin
2019-02-11 17:41     ` Dave Hansen
2019-02-11 17:48       ` Michael S. Tsirkin
2019-02-11 18:30         ` Alexander Duyck
2019-02-11 19:24           ` Michael S. Tsirkin
2019-02-04 18:15 ` [RFC PATCH 3/4] kvm: Add guest " Alexander Duyck
2019-02-04 19:44   ` Dave Hansen
2019-02-04 20:42     ` Alexander Duyck
2019-02-04 23:00   ` Nadav Amit
2019-02-04 23:37     ` Alexander Duyck
2019-02-05  0:03       ` Nadav Amit
2019-02-05  0:16         ` Alexander Duyck
2019-02-05  1:46           ` Nadav Amit
2019-02-05 18:09             ` Alexander Duyck
2019-02-07 18:21   ` Luiz Capitulino
2019-02-07 18:44     ` Alexander Duyck
2019-02-07 20:02       ` Luiz Capitulino
2019-02-08 21:05       ` Nitesh Narayan Lal
2019-02-08 21:31         ` Alexander Duyck [this message]
2019-02-10  0:49   ` Michael S. Tsirkin
2019-02-11 16:31     ` Alexander Duyck
2019-02-11 17:36       ` Michael S. Tsirkin
2019-02-11 18:10         ` Alexander Duyck
2019-02-11 19:54           ` Michael S. Tsirkin
2019-02-11 21:00             ` Alexander Duyck
2019-02-11 22:52               ` Michael S. Tsirkin
     [not found]                 ` <94462313ccd927d25675f69de459456cf066c1a2.camel@linux.intel.com>
2019-02-12  0:34                   ` Michael S. Tsirkin
2019-02-11 17:48     ` Dave Hansen
2019-02-11 17:58       ` Michael S. Tsirkin
2019-02-11 18:19         ` Dave Hansen
2019-02-11 19:56           ` Michael S. Tsirkin
2019-02-04 18:15 ` [RFC PATCH 4/4] mm: Add merge page notifier Alexander Duyck
2019-02-04 19:40   ` Dave Hansen
2019-02-04 19:51     ` Alexander Duyck
2019-02-10  0:57   ` Michael S. Tsirkin
2019-02-11 13:30     ` Nitesh Narayan Lal
2019-02-11 14:17       ` Michael S. Tsirkin
2019-02-11 16:24         ` Nitesh Narayan Lal
2019-02-11 17:41           ` Michael S. Tsirkin
2019-02-11 18:09             ` Nitesh Narayan Lal
2019-02-11  6:40   ` Aaron Lu
2019-02-11 15:58     ` Alexander Duyck
2019-02-12  2:09       ` Aaron Lu
2019-02-12 17:20         ` Alexander Duyck
2019-02-04 18:19 ` [RFC PATCH QEMU] i386/kvm: Enable paravirtual unused page hint mechanism Alexander Duyck
2019-02-05 17:25 ` [RFC PATCH 0/4] kvm: Report unused guest pages to host Nitesh Narayan Lal
2019-02-05 18:43   ` Alexander Duyck
2019-02-07 14:48 ` Nitesh Narayan Lal
2019-02-07 16:56   ` Alexander Duyck
2019-02-10  0:51 ` Michael S. Tsirkin

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