vfio_pin_map_dma cause synchronize_sched wait too long

vfio_pin_map_dma cause synchronize_sched wait too long

[Longpeng (Mike)]

Hi guys,

Suppose there're two VMs: VM1 is bind to node-0 and calling vfio_pin_map_dma(),
VM2 is a migrate incoming VM which bind to node-1. We found the vm_start( QEMU
function) of VM2 will take too long occasionally, the reason is as follow.

- VM2 -
qemu: vm_start
        vm_start_notify
          virtio_vmstate_change
            virtio_pci_vmstate_change
              virtio_pci_start_ioeventfd
                virtio_device_start_ioeventfd_impl
                  event_notifier_init
                    eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC) <-- too long
kern: sys_eventfd2
        get_unused_fd_flags
          __alloc_fd
            expand_files
              expand_fdtable
                synchronize_sched <-- too long

- VM1 -
The VM1 is doing vfio_pin_map_dma at the same time.

The CPU must finish vfio_pin_map_dma and then rcu-sched grace period can be
elapsed, so synchronize_sched would wait for a long time.

Is there any solution to this ? Any suggestion would be greatly appreciated, thanks!

-- 
Regards,
Longpeng(Mike)

Re: vfio_pin_map_dma cause synchronize_sched wait too long

[Paolo Bonzini]

On 02/12/19 10:10, Longpeng (Mike) wrote:
> 
> Suppose there're two VMs: VM1 is bind to node-0 and calling vfio_pin_map_dma(),
> VM2 is a migrate incoming VM which bind to node-1. We found the vm_start( QEMU
> function) of VM2 will take too long occasionally, the reason is as follow.

Which part of vfio_pin_map_dma is running?  There is already a
cond_resched in vfio_iommu_map.  Perhaps you could add one to
vfio_pin_pages_remote and/or use vfio_pgsize_bitmap to cap the number of
pages that it returns.

Paolo

Re: vfio_pin_map_dma cause synchronize_sched wait too long

[Longpeng (Mike)]

在 2019/12/2 17:31, Paolo Bonzini 写道:
> On 02/12/19 10:10, Longpeng (Mike) wrote:
>>
>> Suppose there're two VMs: VM1 is bind to node-0 and calling vfio_pin_map_dma(),
>> VM2 is a migrate incoming VM which bind to node-1. We found the vm_start( QEMU
>> function) of VM2 will take too long occasionally, the reason is as follow.
> 
> Which part of vfio_pin_map_dma is running?  There is already a

I need more analysis to find which part.

> cond_resched in vfio_iommu_map.  Perhaps you could add one to
> vfio_pin_pages_remote and/or use vfio_pgsize_bitmap to cap the number of
> pages that it returns.

Um ... There's only one running task (qemu-kvm of the VM1) on that CPU, so maybe
the cond_resched() is ineffective ?

> > Paolo
> 
> 
> 


-- 
Regards,
Longpeng(Mike)

Re: vfio_pin_map_dma cause synchronize_sched wait too long

[Paolo Bonzini]

On 02/12/19 10:42, Longpeng (Mike) wrote:
>> cond_resched in vfio_iommu_map.  Perhaps you could add one to 
>> vfio_pin_pages_remote and/or use vfio_pgsize_bitmap to cap the
>> number of pages that it returns.
> Um ... There's only one running task (qemu-kvm of the VM1) on that
> CPU, so maybe the cond_resched() is ineffective ?

Note that synchronize_sched() these days is just a synonym of
synchronize_rcu, so this makes me wonder if you're running on an older
kernel and whether you are missing this commit:


commit 92aa39e9dc77481b90cbef25e547d66cab901496
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Mon Jul 9 13:47:30 2018 -0700

    rcu: Make need_resched() respond to urgent RCU-QS needs

    The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
    need for an RCU quiescent state from the force-quiescent-state
processing
    within the grace-period kthread to context switches and to
cond_resched().
    Unfortunately, such urgent needs are not communicated to need_resched(),
    which is sometimes used to decide when to invoke cond_resched(), for
    but one example, within the KVM vcpu_run() function.  As of v4.15, this
    can result in synchronize_sched() being delayed by up to ten seconds,
    which can be problematic, to say nothing of annoying.

    This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
    rcu_check_callbacks(), which is invoked from the scheduling-clock
    interrupt handler.  If the current task is not an idle task and is
    not executing in usermode, a context switch is forced, and either way,
    the rcu_dynticks.rcu_urgent_qs variable is set to false.  If the current
    task is an idle task, then RCU's dyntick-idle code will detect the
    quiescent state, so no further action is required.  Similarly, if the
    task is executing in usermode, other code in rcu_check_callbacks() and
    its called functions will report the corresponding quiescent state.

    Reported-by: Marius Hillenbrand <mhillenb@amazon.de>
    Reported-by: David Woodhouse <dwmw2@infradead.org>
    Suggested-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>


Thanks,

Paolo

Re: vfio_pin_map_dma cause synchronize_sched wait too long

[Longpeng (Mike)]

在 2019/12/2 18:06, Paolo Bonzini 写道:
> On 02/12/19 10:42, Longpeng (Mike) wrote:
>>> cond_resched in vfio_iommu_map.  Perhaps you could add one to 
>>> vfio_pin_pages_remote and/or use vfio_pgsize_bitmap to cap the
>>> number of pages that it returns.
>> Um ... There's only one running task (qemu-kvm of the VM1) on that
>> CPU, so maybe the cond_resched() is ineffective ?
> 
> Note that synchronize_sched() these days is just a synonym of
> synchronize_rcu, so this makes me wonder if you're running on an older
> kernel and whether you are missing this commit:
> 

Yep. I'm running on an older kernel and I've missed this patchset. Thanks a lot :)

> 
> commit 92aa39e9dc77481b90cbef25e547d66cab901496
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date:   Mon Jul 9 13:47:30 2018 -0700
> 
>     rcu: Make need_resched() respond to urgent RCU-QS needs
> 
>     The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
>     need for an RCU quiescent state from the force-quiescent-state
> processing
>     within the grace-period kthread to context switches and to
> cond_resched().
>     Unfortunately, such urgent needs are not communicated to need_resched(),
>     which is sometimes used to decide when to invoke cond_resched(), for
>     but one example, within the KVM vcpu_run() function.  As of v4.15, this
>     can result in synchronize_sched() being delayed by up to ten seconds,
>     which can be problematic, to say nothing of annoying.
> 
>     This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
>     rcu_check_callbacks(), which is invoked from the scheduling-clock
>     interrupt handler.  If the current task is not an idle task and is
>     not executing in usermode, a context switch is forced, and either way,
>     the rcu_dynticks.rcu_urgent_qs variable is set to false.  If the current
>     task is an idle task, then RCU's dyntick-idle code will detect the
>     quiescent state, so no further action is required.  Similarly, if the
>     task is executing in usermode, other code in rcu_check_callbacks() and
>     its called functions will report the corresponding quiescent state.
> 
>     Reported-by: Marius Hillenbrand <mhillenb@amazon.de>
>     Reported-by: David Woodhouse <dwmw2@infradead.org>
>     Suggested-by: Peter Zijlstra <peterz@infradead.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> 
> Thanks,
> 
> Paolo
> 
> 
> .
> 


-- 
Regards,
Longpeng(Mike)