Re: [PATCH v2] KVM: Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG

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On 2024/4/3 上午5:36, David Matlack wrote:
Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG to avoid
blocking other threads (e.g. vCPUs taking page faults) for too long.

Specifically, change kvm_clear_dirty_log_protect() to acquire/release
mmu_lock only when calling kvm_arch_mmu_enable_log_dirty_pt_masked(),
rather than around the entire for loop. This ensures that KVM will only
hold mmu_lock for the time it takes the architecture-specific code to
process up to 64 pages, rather than holding mmu_lock for log->num_pages,
which is controllable by userspace. This also avoids holding mmu_lock
when processing parts of the dirty_bitmap that are zero (i.e. when there
is nothing to clear).

Moving the acquire/release points for mmu_lock should be safe since
dirty_bitmap_buffer is already protected by slots_lock, and dirty_bitmap
is already accessed with atomic_long_fetch_andnot(). And at least on x86
holding mmu_lock doesn't even serialize access to the memslot dirty
bitmap, as vCPUs can call mark_page_dirty_in_slot() without holding

This change eliminates dips in guest performance during live migration
in a 160 vCPU VM when userspace is issuing CLEAR ioctls (tested with
1GiB and 8GiB CLEARs). Userspace could issue finer-grained CLEARs, which
Frequently drop/reacquire mmu_lock will cause userspace migration process issuing CLEAR ioctls to contend with 160 vCPU, migration speed maybe become slower. In theory priority of userspace migration thread should be higher than vcpu thread.

Drop and reacquire mmu_lock with 64-pages may be a little too smaller,
in generic it is one huge page size. However it should be decided by framework maintainer:)

Bibo Mao
would also reduce contention on mmu_lock, but doing so will increase the
rate of remote TLB flushing. And there's really no reason to punt this
problem to userspace since KVM can just drop and reacquire mmu_lock more

Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
Cc: Tianrui Zhao <zhaotianrui@xxxxxxxxxxx>
Cc: Bibo Mao <maobibo@xxxxxxxxxxx>
Cc: Huacai Chen <chenhuacai@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Anup Patel <anup@xxxxxxxxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Cc: Janosch Frank <frankja@xxxxxxxxxxxxx>
Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
  - Rebase onto kvm/queue [Marc]


  virt/kvm/kvm_main.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index fb49c2a60200..0a8b25a52c15 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2386,7 +2386,6 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
  	if (copy_from_user(dirty_bitmap_buffer, log->dirty_bitmap, n))
  		return -EFAULT;
- KVM_MMU_LOCK(kvm);
  	for (offset = log->first_page, i = offset / BITS_PER_LONG,
  		 n = DIV_ROUND_UP(log->num_pages, BITS_PER_LONG); n--;
  	     i++, offset += BITS_PER_LONG) {
@@ -2405,11 +2404,12 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
  		if (mask) {
  			flush = true;
+			KVM_MMU_LOCK(kvm);
  			kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
  								offset, mask);
+			KVM_MMU_UNLOCK(kvm);
if (flush)
  		kvm_flush_remote_tlbs_memslot(kvm, memslot);

base-commit: 9bc60f733839ab6fcdde0d0b15cbb486123e6402

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