[0/2] KVM: x86/mmu: Run NX huge page recovery under MMU read lock

Message

Vipin Sharma Aug. 12, 2024, 5:13 p.m. UTC

Split NX huge page recovery in two separate flows, one for TDP MMU and
one for non-TDP MMU.

TDP MMU flow will use MMU read lock and non-TDP MMU flow will use MMU
write lock. This change unblocks vCPUs which are waiting for MMU read
lock while NX huge page recovery is running and zapping shadow pages.

A Windows guest was showing network latency jitters which was root
caused to vCPUs waiting for MMU read lock when NX huge page recovery
thread was holding MMU write lock. Disabling NX huge page recovery fixed
the jitter issue.

So, to optimize NX huge page recovery, it was modified to run under MMU
read lock, the switch made jitter issue disappear completely and vCPUs
wait time for MMU read lock reduced drastically.

Patch 1 is splitting the logic in two separate flows but still running
under MMU write lock.

Patch 2 is changing TDP MMU flow to use MMU read lock.

Patch 2 commit log contains the test results. Here is the brief
histogram, where 'Interval' is the time it took to complete the network
calls and 'Frequency' is how many calls:

Before
------

Interval(usec)   Frequency
      0          9999964
   1000          12
   2000          3
   3000          0
   4000          0
   5000          0
   6000          0
   7000          1
   8000          1
   9000          1
  10000          2
  11000          1
  12000          0
  13000          4
  14000          1
  15000          1
  16000          4
  17000          1
  18000          2
  19000          0
  20000          0
  21000          1
  22000          0
  23000          0
  24000          1

After
-----

Interval(usec)   Frequency
      0          9999996
   1000          4

Vipin Sharma (2):
  KVM: x86/mmu: Split NX hugepage recovery flow into TDP and non-TDP
    flow
  KVM: x86/mmu: Recover NX Huge pages belonging to TDP MMU under MMU
    read lock

 arch/x86/kvm/mmu/mmu.c          | 168 +++++++++++++++++++-------------
 arch/x86/kvm/mmu/mmu_internal.h |   6 ++
 arch/x86/kvm/mmu/tdp_mmu.c      |  89 +++++++++++++++--
 arch/x86/kvm/mmu/tdp_mmu.h      |   3 +-
 4 files changed, 192 insertions(+), 74 deletions(-)


base-commit: 332d2c1d713e232e163386c35a3ba0c1b90df83f