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Re: vfio_pin_map_dma cause synchronize_sched wait too long

From: Longpeng (Mike)
Subject: Re: vfio_pin_map_dma cause synchronize_sched wait too long
Date: Mon, 2 Dec 2019 18:47:11 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

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 <address@hidden>
> 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 <address@hidden>
>     Reported-by: David Woodhouse <address@hidden>
>     Suggested-by: Peter Zijlstra <address@hidden>
>     Signed-off-by: Paul E. McKenney <address@hidden>
> Thanks,
> Paolo
> .


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