linux-kernel - [PATCH v3 5/7] KVM: X86: MMU: Tune PTE_LIST

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Date:   Fri, 30 Jul 2021 18:04:53 -0400
From:   Peter Xu <peterx@...hat.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, peterx@...hat.com,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH v3 5/7] KVM: X86: MMU: Tune PTE_LIST_EXT to be bigger

Currently rmap array element only contains 3 entries.  However for EPT=N there
could have a lot of guest pages that got tens of even hundreds of rmap entry.

A normal distribution of a 6G guest (even if idle) shows this with rmap count
statistics:

Rmap_Count:     0       1       2-3     4-7     8-15    16-31   32-63   64-127  128-255 256-511 512-1023
Level=4K:       3089171 49005   14016   1363    235     212     15      7       0       0       0
Level=2M:       5951    227     0       0       0       0       0       0       0       0       0
Level=1G:       32      0       0       0       0       0       0       0       0       0       0

If we do some more fork some pages will grow even larger rmap counts.

This patch makes PTE_LIST_EXT bigger so it'll be more efficient for the general
use case of EPT=N as we do list reference less and the loops over PTE_LIST_EXT
will be slightly more efficient; but still not too large so less waste when
array not full.

It should not affecting EPT=Y since EPT normally only has zero or one rmap
entry for each page, so no array is even allocated.

With a test case to fork 500 child and recycle them ("./rmap_fork 500" [1]),
this patch speeds up fork time of about 29%.

    Before: 473.90 (+-5.93%)
    After:  366.10 (+-4.94%)

[1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 arch/x86/kvm/mmu/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 16c99f771c9e..c0b452bb5dd9 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -137,8 +137,8 @@ module_param(dbg, bool, 0644);

 #include <trace/events/kvm.h>

-/* make pte_list_desc fit well in cache line */
-#define PTE_LIST_EXT 3
+/* make pte_list_desc fit well in cache lines */
+#define PTE_LIST_EXT 15

 struct pte_list_desc {
 	u64 *sptes[PTE_LIST_EXT];
-- 
2.31.1