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[184.144.111.238]) by smtp.gmail.com with ESMTPSA id a18sm2355666ilc.31.2021.06.24.15.53.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Jun 2021 15:53:50 -0700 (PDT) Date: Thu, 24 Jun 2021 18:53:49 -0400 From: Peter Xu To: kvm@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Paolo Bonzini , Vitaly Kuznetsov , Maxim Levitsky , Sean Christopherson Subject: Re: [PATCH 8/9] KVM: X86: Optimize pte_list_desc with per-array counter Message-ID: References: <20210624181356.10235-1-peterx@redhat.com> <20210624181520.11012-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210624181520.11012-1-peterx@redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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 > --- > 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 > > /* 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