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SubjectRe: vfio_pin_map_dma cause synchronize_sched wait too long
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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)

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