linux-kernel - [PATCH] KVM: VMX: Optimize flushing the PML buffer

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Date:   Thu,  4 Feb 2021 14:19:59 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH] KVM: VMX: Optimize flushing the PML buffer

vmx_flush_pml_buffer repeatedly calls kvm_vcpu_mark_page_dirty, which
SRCU-derefrences kvm->memslots. In order to give the compiler more
freedom to optimize the function, SRCU-dereference the pointer
kvm->memslots only once.

Reviewed-by: Makarand Sonare <makarandsonare@...gle.com>
Signed-off-by: Ben Gardon <bgardon@...gle.com>

---

Tested by running the dirty_log_perf_test selftest on a dual socket Intel
Skylake machine:
./dirty_log_perf_test -v 4 -b 30G -i 5

The test was run 5 times with and without this patch and the dirty
memory time for iterations 2-5 was averaged across the 5 runs.
Iteration 1 was discarded for this analysis because it is still dominated
by the time spent populating memory.

The average time for each run demonstrated a strange bimodal distribution,
with clusters around 2 seconds and 2.5 seconds. This may have been a
result of vCPU migration between NUMA nodes.

In any case, the get dirty times with this patch averaged to 2.07
seconds, a 7% savings from the 2.22 second everage without this patch.

While these savings may be partly a result of the patched runs having
one more 2 second clustered run, the patched runs in the higer cluster
were also 7-8% shorter than those in the unpatched case.

Below is the raw data for anyone interested in visualizing the results
with a graph:
Iteration	Baseline	Patched
2		2.038562907	2.045226614
3		2.037363248	2.045033709
4		2.037176331	1.999783966
5		1.999891981	2.007849104
2		2.569526298	2.001252504
3		2.579110209	2.008541897
4		2.585883731	2.005317983
5		2.588692727	2.007100987
2		2.01191437	2.006953735
3		2.012972236	2.04540153
4		1.968836017	2.005035246
5		1.967915154	2.003859551
2		2.037533296	1.991275846
3		2.501480125	2.391886691
4		2.454382587	2.391904789
5		2.461046772	2.398767963
2		2.036991484	2.011331436
3		2.002954418	2.002635687
4		2.053342717	2.006769959
5		2.522539759	2.006470059
Average		2.223405818	2.069119963

 arch/x86/kvm/vmx/vmx.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index cc60b1fc3ee7..46c54802dfdb 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5692,6 +5692,7 @@ static void vmx_destroy_pml_buffer(struct vcpu_vmx *vmx)
 static void vmx_flush_pml_buffer(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	struct kvm_memslots *memslots;
 	u64 *pml_buf;
 	u16 pml_idx;
 
@@ -5707,13 +5708,18 @@ static void vmx_flush_pml_buffer(struct kvm_vcpu *vcpu)
 	else
 		pml_idx++;
 
+	memslots = kvm_vcpu_memslots(vcpu);
+
 	pml_buf = page_address(vmx->pml_pg);
 	for (; pml_idx < PML_ENTITY_NUM; pml_idx++) {
+		struct kvm_memory_slot *memslot;
 		u64 gpa;
 
 		gpa = pml_buf[pml_idx];
 		WARN_ON(gpa & (PAGE_SIZE - 1));
-		kvm_vcpu_mark_page_dirty(vcpu, gpa >> PAGE_SHIFT);
+
+		memslot = __gfn_to_memslot(memslots, gpa >> PAGE_SHIFT);
+		mark_page_dirty_in_slot(vcpu->kvm, memslot, gpa >> PAGE_SHIFT);
 	}
 
 	/* reset PML index */
-- 
2.30.0.365.g02bc693789-goog