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SubjectRe: [PATCH 7/8] KVM: make async_pf work queue lockless
On Wed, Oct 27, 2010 at 05:09:41PM +0800, Xiao Guangrong wrote:
> The async_pf number is very few since only pending interrupt can
> let it re-enter to the guest mode.
>
> During my test(Host 4 CPU + 4G, Guest 4 VCPU + 6G), it's no
> more than 10 requests in the system.
>
> So, we can only increase the completion counter in the work queue
> context, and walk vcpu->async_pf.queue list to get all completed
> async_pf
>
That depends on the load. I used memory cgroups to create very big
memory pressure and I saw hundreds of apfs per second. We shouldn't
optimize for very low numbers. With vcpu->async_pf.queue having more
then one element I am not sure your patch is beneficial.

> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> ---
> include/linux/kvm_host.h | 4 +--
> virt/kvm/async_pf.c | 50 +++++++++++++++++++--------------------------
> 2 files changed, 22 insertions(+), 32 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index d91add9..33c03c3 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -78,7 +78,6 @@ int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> #ifdef CONFIG_KVM_ASYNC_PF
> struct kvm_async_pf {
> struct work_struct work;
> - struct list_head link;
> struct list_head queue;
> struct kvm_vcpu *vcpu;
> struct mm_struct *mm;
> @@ -127,10 +126,9 @@ struct kvm_vcpu {
>
> #ifdef CONFIG_KVM_ASYNC_PF
> struct {
> + atomic_t done;
> u32 queued;
> struct list_head queue;
> - struct list_head done;
> - spinlock_t lock;
> bool wakeup;
> } async_pf;
> #endif
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index 0d1f6c4..f10de1e 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -49,9 +49,7 @@ void kvm_async_pf_deinit(void)
>
> void kvm_async_pf_vcpu_init(struct kvm_vcpu *vcpu)
> {
> - INIT_LIST_HEAD(&vcpu->async_pf.done);
> INIT_LIST_HEAD(&vcpu->async_pf.queue);
> - spin_lock_init(&vcpu->async_pf.lock);
> }
>
> static void async_pf_execute(struct work_struct *work)
> @@ -72,11 +70,9 @@ static void async_pf_execute(struct work_struct *work)
> up_read(&mm->mmap_sem);
> unuse_mm(mm);
>
> - spin_lock(&vcpu->async_pf.lock);
> - list_add_tail(&apf->link, &vcpu->async_pf.done);
> apf->page = page;
> apf->done = true;
> - spin_unlock(&vcpu->async_pf.lock);
> + atomic_inc(&vcpu->async_pf.done);
>
> /*
> * apf may be freed by kvm_check_async_pf_completion() after
> @@ -101,52 +97,48 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
> typeof(*work), queue);
> cancel_work_sync(&work->work);
> list_del(&work->queue);
> - if (!work->done) /* work was canceled */
> - kmem_cache_free(async_pf_cache, work);
> - }
> -
> - spin_lock(&vcpu->async_pf.lock);
> - while (!list_empty(&vcpu->async_pf.done)) {
> - struct kvm_async_pf *work =
> - list_entry(vcpu->async_pf.done.next,
> - typeof(*work), link);
> - list_del(&work->link);
> if (work->page)
> put_page(work->page);
> kmem_cache_free(async_pf_cache, work);
> }
> - spin_unlock(&vcpu->async_pf.lock);
>
> vcpu->async_pf.queued = 0;
> + atomic_set(&vcpu->async_pf.done, 0);
> }
>
> bool kvm_check_async_pf_completion(struct kvm_vcpu *vcpu)
> {
> struct kvm_async_pf *work;
> + struct list_head *pos, *temp;
>
> if (vcpu->async_pf.wakeup) {
> vcpu->async_pf.wakeup = false;
> return true;
> }
>
> - if (list_empty_careful(&vcpu->async_pf.done) ||
> + if (!atomic_read(&vcpu->async_pf.done) ||
> !kvm_arch_can_inject_async_page_present(vcpu))
> return false;
>
> - spin_lock(&vcpu->async_pf.lock);
> - work = list_first_entry(&vcpu->async_pf.done, typeof(*work), link);
> - list_del(&work->link);
> - spin_unlock(&vcpu->async_pf.lock);
> + list_for_each_safe(pos, temp, &vcpu->async_pf.queue) {
> + work = list_entry(pos, typeof(*work), queue);
> + if (!work->done)
> + continue;
>
> - if (work->page)
> - kvm_arch_async_page_ready(vcpu, work);
> - kvm_arch_async_page_present(vcpu, work);
> + if (work->page) {
> + kvm_arch_async_page_ready(vcpu, work);
> + put_page(work->page);
> + }
> +
> + kvm_arch_async_page_present(vcpu, work);
> +
> + list_del(&work->queue);
> + vcpu->async_pf.queued--;
> + kmem_cache_free(async_pf_cache, work);
> + if (atomic_dec_and_test(&vcpu->async_pf.done))
> + break;
You should do atomic_dec() and always break. We cannot inject two apfs during
one vcpu entry.

> + }
>
> - list_del(&work->queue);
> - vcpu->async_pf.queued--;
> - if (work->page)
> - put_page(work->page);
> - kmem_cache_free(async_pf_cache, work);
> kvm_arch_async_pf_completion(vcpu);
>
> return true;
> --
> 1.7.0.4
>
> --
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--
Gleb.


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