[PATCH v4] KVM: async_pf: make rcu irq exit if not triggered from idle task

[PATCH v4] KVM: async_pf: make rcu irq exit if not triggered from idle task

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@hotmail.com>

 WARNING: CPU: 5 PID: 1242 at kernel/rcu/tree_plugin.h:323 rcu_note_context_switch+0x207/0x6b0
 CPU: 5 PID: 1242 Comm: unity-settings- Not tainted 4.13.0-rc2+ #1
 RIP: 0010:rcu_note_context_switch+0x207/0x6b0
 Call Trace:
  __schedule+0xda/0xba0
  ? kvm_async_pf_task_wait+0x1b2/0x270
  schedule+0x40/0x90
  kvm_async_pf_task_wait+0x1cc/0x270
  ? prepare_to_swait+0x22/0x70
  do_async_page_fault+0x77/0xb0
  ? do_async_page_fault+0x77/0xb0
  async_page_fault+0x28/0x30
 RIP: 0010:__d_lookup_rcu+0x90/0x1e0

I encounter this when trying to stress the async page fault in L1 guest w/ 
L2 guests running.

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 patch fixes it by informing the RCU subsystem exit/enter the irq 
towards/away from idle for both n.halted and !n.halted.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v3 -> v4:
 * pull rcu_irq_exit/enter outside the "if"
v2 -> v3: 
 * add the warning log
v1 -> v2:
 * update patch description

 arch/x86/kernel/kvm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

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);
-- 
2.7.4

Re: [PATCH v4] KVM: async_pf: make rcu irq exit if not triggered from idle task

[Paolo Bonzini]

On 01/08/2017 14:20, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
> 
>  WARNING: CPU: 5 PID: 1242 at kernel/rcu/tree_plugin.h:323 rcu_note_context_switch+0x207/0x6b0
>  CPU: 5 PID: 1242 Comm: unity-settings- Not tainted 4.13.0-rc2+ #1
>  RIP: 0010:rcu_note_context_switch+0x207/0x6b0
>  Call Trace:
>   __schedule+0xda/0xba0
>   ? kvm_async_pf_task_wait+0x1b2/0x270
>   schedule+0x40/0x90
>   kvm_async_pf_task_wait+0x1cc/0x270
>   ? prepare_to_swait+0x22/0x70
>   do_async_page_fault+0x77/0xb0
>   ? do_async_page_fault+0x77/0xb0
>   async_page_fault+0x28/0x30
>  RIP: 0010:__d_lookup_rcu+0x90/0x1e0
> 
> I encounter this when trying to stress the async page fault in L1 guest w/ 
> L2 guests running.
> 
> 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 not accurate anymore:

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.  However,
we need to call the pair even if the function calls schedule(), as seen
from the above backtrace.

Can be fixed when committing.

> This patch fixes it by informing the RCU subsystem exit/enter the irq 
> towards/away from idle for both n.halted and !n.halted.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
> v3 -> v4:
>  * pull rcu_irq_exit/enter outside the "if"
> v2 -> v3: 
>  * add the warning log
> v1 -> v2:
>  * update patch description
> 
>  arch/x86/kernel/kvm.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> 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);
> 

Apart from that,

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Re: [PATCH v4] KVM: async_pf: make rcu irq exit if not triggered from idle task

[Radim Krčmář]

2017-08-01 14:22+0200, Paolo Bonzini:
> On 01/08/2017 14:20, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpeng.li@hotmail.com>
> > 
> >  WARNING: CPU: 5 PID: 1242 at kernel/rcu/tree_plugin.h:323 rcu_note_context_switch+0x207/0x6b0
> >  CPU: 5 PID: 1242 Comm: unity-settings- Not tainted 4.13.0-rc2+ #1
> >  RIP: 0010:rcu_note_context_switch+0x207/0x6b0
> >  Call Trace:
> >   __schedule+0xda/0xba0
> >   ? kvm_async_pf_task_wait+0x1b2/0x270
> >   schedule+0x40/0x90
> >   kvm_async_pf_task_wait+0x1cc/0x270
> >   ? prepare_to_swait+0x22/0x70
> >   do_async_page_fault+0x77/0xb0
> >   ? do_async_page_fault+0x77/0xb0
> >   async_page_fault+0x28/0x30
> >  RIP: 0010:__d_lookup_rcu+0x90/0x1e0
> > 
> > I encounter this when trying to stress the async page fault in L1 guest w/ 
> > L2 guests running.
> > 
> > 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 not accurate anymore:
> 
> 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.  However,
> we need to call the pair even if the function calls schedule(), as seen
> from the above backtrace.
> 
> Can be fixed when committing.

Done that, thanks.