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Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle task

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  Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle  task Paolo Bonzini <pbonzini@redhat.com> - 2017-08-01 12:40 +0200
    Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle task Wanpeng Li <kernellwp@gmail.com> - 2017-08-01 13:20 +0200
      Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle  task Paolo Bonzini <pbonzini@redhat.com> - 2017-08-01 13:40 +0200

#1700826 — Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle task

On 27/07/2017 11:05, Wanpeng Li wrote:
> Commit 9b132fbe5419 (Add rcu user eqs exception hooks for async page fault) 
> adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu idle eqs 
> when needed, to protect the code that needs use rcu. There is no need to call 
> this pairs if async page fault is not triggered from idle task.

This is true, but I think the patch is making things more complex where
it could be simplifying them.  Right now, the "native_safe_halt" branch
is calling rcu_irq_exit/enter but the "schedule" branch is not.  Could
you just pull rcu_irq_exit/enter outside the "if", so that you inform
the RCU subsystem even in the !n.halted case?

Paolo

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#1700886 — Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle task

2017-08-01 18:37 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
> On 27/07/2017 11:05, Wanpeng Li wrote:
>> Commit 9b132fbe5419 (Add rcu user eqs exception hooks for async page fault)
>> adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu idle eqs
>> when needed, to protect the code that needs use rcu. There is no need to call
>> this pairs if async page fault is not triggered from idle task.
>
> This is true, but I think the patch is making things more complex where
> it could be simplifying them.  Right now, the "native_safe_halt" branch
> is calling rcu_irq_exit/enter but the "schedule" branch is not.  Could
> you just pull rcu_irq_exit/enter outside the "if", so that you inform
> the RCU subsystem even in the !n.halted case?

How about something like this?

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 71c17a5..d04e30e 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -151,6 +151,8 @@ void kvm_async_pf_task_wait(u32 token)
         if (hlist_unhashed(&n.link))
             break;

+        rcu_irq_exit();
+
         if (!n.halted) {
             local_irq_enable();
             schedule();
@@ -159,11 +161,11 @@ void kvm_async_pf_task_wait(u32 token)
             /*
              * We cannot reschedule. So halt.
              */
-            rcu_irq_exit();
             native_safe_halt();
             local_irq_disable();
-            rcu_irq_enter();
         }
+
+        rcu_irq_enter();
     }
     if (!n.halted)
         finish_swait(&n.wq, &wait);

Regards,
Wanpeng Li

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#1700913

On 01/08/2017 13:13, Wanpeng Li wrote:
> 2017-08-01 18:37 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>> On 27/07/2017 11:05, Wanpeng Li wrote:
>>> Commit 9b132fbe5419 (Add rcu user eqs exception hooks for async page fault)
>>> adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu idle eqs
>>> when needed, to protect the code that needs use rcu. There is no need to call
>>> this pairs if async page fault is not triggered from idle task.
>>
>> This is true, but I think the patch is making things more complex where
>> it could be simplifying them.  Right now, the "native_safe_halt" branch
>> is calling rcu_irq_exit/enter but the "schedule" branch is not.  Could
>> you just pull rcu_irq_exit/enter outside the "if", so that you inform
>> the RCU subsystem even in the !n.halted case?
> 
> How about something like this?

If it works, it's perfect. :)

Please add Cc for stable kernels too in v4.

Paolo

> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 71c17a5..d04e30e 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -151,6 +151,8 @@ void kvm_async_pf_task_wait(u32 token)
>          if (hlist_unhashed(&n.link))
>              break;
> 
> +        rcu_irq_exit();
> +
>          if (!n.halted) {
>              local_irq_enable();
>              schedule();
> @@ -159,11 +161,11 @@ void kvm_async_pf_task_wait(u32 token)
>              /*
>               * We cannot reschedule. So halt.
>               */
> -            rcu_irq_exit();
>              native_safe_halt();
>              local_irq_disable();
> -            rcu_irq_enter();
>          }
> +
> +        rcu_irq_enter();
>      }
>      if (!n.halted)
>          finish_swait(&n.wq, &wait);
> 
> Regards,
> Wanpeng Li
> 

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