From: "Longpeng (Mike)" <longpeng2@huawei.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
Alex Williamson <alex.williamson@redhat.com>
Cc: <qemu-devel@nongnu.org>, <kvm@vger.kernel.org>,
<linux-kernel@vger.kernel.org>,
"Longpeng(Mike)" <longpeng.mike@gmail.com>,
Gonglei <arei.gonglei@huawei.com>,
Huangzhichao <huangzhichao@huawei.com>
Subject: Re: vfio_pin_map_dma cause synchronize_sched wait too long
Date: Mon, 2 Dec 2019 18:47:11 +0800 [thread overview]
Message-ID: <8a14a25d-61ce-5fb3-edc2-2c69b18f8e36@huawei.com> (raw)
In-Reply-To: <42c907fd-6c09-fbb6-d166-60e6827edff5@redhat.com>
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)
prev parent reply other threads:[~2019-12-02 10:47 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-02 9:10 vfio_pin_map_dma cause synchronize_sched wait too long Longpeng (Mike)
2019-12-02 9:31 ` Paolo Bonzini
2019-12-02 9:42 ` Longpeng (Mike)
2019-12-02 10:06 ` Paolo Bonzini
2019-12-02 10:47 ` Longpeng (Mike) [this message]
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