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

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Date:   Tue, 1 Aug 2017 19:13:24 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: 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@...hat.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