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Date: Thu, 24 Jun 2021 18:53:49 -0400
From: Peter Xu <peterx@...hat.com>
To: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Maxim Levitsky <mlevitsk@...hat.com>,
Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 8/9] KVM: X86: Optimize pte_list_desc with per-array
counter
On Thu, Jun 24, 2021 at 02:15:20PM -0400, Peter Xu wrote:
> Add a counter field into pte_list_desc, so as to simplify the add/remove/loop
> logic. E.g., we don't need to loop over the array any more for most reasons.
>
> This will make more sense after we've switched the array size to be larger
> otherwise the counter will be a waste.
>
> Initially I wanted to store a tail pointer at the head of the array list so we
> don't need to traverse the list at least for pushing new ones (if without the
> counter we traverse both the list and the array). However that'll need
> slightly more change without a huge lot benefit, e.g., after we grow entry
> numbers per array the list traversing is not so expensive.
>
> So let's be simple but still try to get as much benefit as we can with just
> these extra few lines of changes (not to mention the code looks easier too
> without looping over arrays).
>
> I used the same a test case to fork 500 child and recycle them ("./rmap_fork
> 500" [1]), this patch further speeds up the total fork time of about 14%, which
> is a total of 38% of vanilla kernel:
>
> Vanilla: 367.20 (+-4.58%)
> 3->15 slots: 302.00 (+-5.30%)
> Add counter: 265.20 (+-9.88%)
>
> [1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 26 ++++++++++++++++----------
> 1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 8888ae291cb9..b21e52dfc27b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -136,10 +136,15 @@ module_param(dbg, bool, 0644);
> #include <trace/events/kvm.h>
>
> /* make pte_list_desc fit well in cache lines */
> -#define PTE_LIST_EXT 15
> +#define PTE_LIST_EXT 14
>
> struct pte_list_desc {
> u64 *sptes[PTE_LIST_EXT];
> + /*
> + * Stores number of entries stored in the pte_list_desc. No need to be
> + * u64 but just for easier alignment. When PTE_LIST_EXT, means full.
> + */
> + u64 spte_count;
> struct pte_list_desc *more;
> };
>
> @@ -830,7 +835,7 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
> struct kvm_rmap_head *rmap_head)
> {
> struct pte_list_desc *desc;
> - int i, count = 0;
> + int count = 0;
>
> if (!rmap_head->val) {
> rmap_printk("%p %llx 0->1\n", spte, *spte);
> @@ -840,24 +845,24 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
> desc = mmu_alloc_pte_list_desc(vcpu);
> desc->sptes[0] = (u64 *)rmap_head->val;
> desc->sptes[1] = spte;
> + desc->spte_count = 2;
> rmap_head->val = (unsigned long)desc | 1;
> ++count;
> } else {
> rmap_printk("%p %llx many->many\n", spte, *spte);
> desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
> - while (desc->sptes[PTE_LIST_EXT-1]) {
> + while (desc->spte_count == PTE_LIST_EXT) {
> count += PTE_LIST_EXT;
> -
> if (!desc->more) {
> desc->more = mmu_alloc_pte_list_desc(vcpu);
> desc = desc->more;
> + desc->spte_count = 0;
> break;
> }
> desc = desc->more;
> }
> - for (i = 0; desc->sptes[i]; ++i)
> - ++count;
> - desc->sptes[i] = spte;
> + count += desc->spte_count;
> + desc->sptes[desc->spte_count++] = spte;
> }
> return count;
> }
> @@ -873,8 +878,10 @@ pte_list_desc_remove_entry(struct kvm_rmap_head *rmap_head,
> ;
> desc->sptes[i] = desc->sptes[j];
> desc->sptes[j] = NULL;
> + desc->spte_count--;
> if (j != 0)
> return;
> + WARN_ON_ONCE(desc->spte_count);
> if (!prev_desc && !desc->more)
> rmap_head->val = 0;
> else
> @@ -930,7 +937,7 @@ static void pte_list_remove(struct kvm_rmap_head *rmap_head, u64 *sptep)
> unsigned int pte_list_count(struct kvm_rmap_head *rmap_head)
> {
> struct pte_list_desc *desc;
> - unsigned int i, count = 0;
> + unsigned int count = 0;
>
> if (!rmap_head->val)
> return 0;
> @@ -940,8 +947,7 @@ unsigned int pte_list_count(struct kvm_rmap_head *rmap_head)
> desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
>
> while (desc) {
> - for (i = 0; (i < PTE_LIST_EXT) && desc->sptes[i]; i++)
> - count++;
> + count += desc->spte_count;
> desc = desc->more;
> }
I think I still missed another loop in pte_list_desc_remove_entry() that we can
drop. With some other cleanups, I plan to squash below into this patch too..
---8<---
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 719fb6fd0aa0..2d8c56eb36f8 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -872,16 +872,13 @@ pte_list_desc_remove_entry(struct kvm_rmap_head *rmap_head,
struct pte_list_desc *desc, int i,
struct pte_list_desc *prev_desc)
{
- int j;
+ int j = desc->spte_count - 1;
- for (j = PTE_LIST_EXT - 1; !desc->sptes[j] && j > i; --j)
- ;
desc->sptes[i] = desc->sptes[j];
desc->sptes[j] = NULL;
desc->spte_count--;
- if (j != 0)
+ if (desc->spte_count)
return;
- WARN_ON_ONCE(desc->spte_count);
if (!prev_desc && !desc->more)
rmap_head->val = 0;
else
@@ -913,7 +910,7 @@ static void __pte_list_remove(u64 *spte, struct kvm_rmap_head *rmap_head)
desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
prev_desc = NULL;
while (desc) {
- for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i) {
+ for (i = 0; i < desc->spte_count; ++i) {
if (desc->sptes[i] == spte) {
pte_list_desc_remove_entry(rmap_head,
desc, i, prev_desc);
---8<---
--
Peter Xu
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