[PATCH 7/9] KVM: X86: MMU: Tune PTE_LIST_EXT to be bigger

From: Peter Xu
Date: Thu Jun 24 2021 - 14:14:23 EST


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 22%.

Before: 367.20 (+-4.58%)
After: 302.00 (+-5.30%)

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

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
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 80263ecb1de3..8888ae291cb9 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -135,8 +135,8 @@ 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