On 04/26/2010 12:58 PM, Takuya Yoshikawa wrote:
Although we always allocate a new dirty bitmap in x86's get_dirty_log(),
it is only used as a zero-source of copy_to_user() and freed right after
that when memslot is clean. This patch uses clear_user() instead of doing
this unnecessary zero-source allocation.
Performance improvement: as we can expect easily, the time needed to
allocate a bitmap is completely reduced. In my test, the improved ioctl
was about 4 to 10 times faster than the original one for clean slots.
Furthermore, the reduced allocations seem to produce good effects for
other cases too. Actually, I observed that the time for the ioctl was
more stable than the original one and the average time for dirty slots
was also reduced by some extent.
Can you explain why the dirty slots were improved?
Signed-off-by: Takuya Yoshikawa<yoshikawa.takuya@xxxxxxxxxxxxx>
---
arch/x86/kvm/x86.c | 36 ++++++++++++++++++++++--------------
1 files changed, 22 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6b2ce1d..0086d64 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2744,7 +2744,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
struct kvm_memory_slot *memslot;
unsigned long n;
unsigned long is_dirty = 0;
- unsigned long *dirty_bitmap = NULL;
mutex_lock(&kvm->slots_lock);
@@ -2759,27 +2758,29 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
n = kvm_dirty_bitmap_bytes(memslot);
- r = -ENOMEM;
- dirty_bitmap = vmalloc(n);
- if (!dirty_bitmap)
- goto out;
- memset(dirty_bitmap, 0, n);
-
for (i = 0; !is_dirty&& i< n/sizeof(long); i++)
is_dirty = memslot->dirty_bitmap[i];
/* If nothing is dirty, don't bother messing with page tables. */
if (is_dirty) {
struct kvm_memslots *slots, *old_slots;
+ unsigned long *dirty_bitmap;
spin_lock(&kvm->mmu_lock);
kvm_mmu_slot_remove_write_access(kvm, log->slot);
spin_unlock(&kvm->mmu_lock);
- slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
- if (!slots)
- goto out_free;
+ r = -ENOMEM;
+ dirty_bitmap = vmalloc(n);
+ if (!dirty_bitmap)
+ goto out;
+ memset(dirty_bitmap, 0, n);
Better to keep r = -ENOMEM here, so if something else is inserted, we
don't lose the error. It logically belongs with the allocation, not
inherited.
+ slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
+ if (!slots) {
+ vfree(dirty_bitmap);
+ goto out;
+ }
memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
slots->memslots[log->slot].dirty_bitmap = dirty_bitmap;
@@ -2788,13 +2789,20 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
synchronize_srcu_expedited(&kvm->srcu);
dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
kfree(old_slots);
+
+ r = -EFAULT;
+ if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
+ vfree(dirty_bitmap);
What about slots? ah, I see they were already freed.
+ goto out;
+ }
+ vfree(dirty_bitmap);
+ } else {
+ r = -EFAULT;
+ if (clear_user(log->dirty_bitmap, n))
+ goto out;
}
r = 0;
- if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n))
- r = -EFAULT;
-out_free:
- vfree(dirty_bitmap);
out:
mutex_unlock(&kvm->slots_lock);
return r;
--
error compiling committee.c: too many arguments to function
--
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