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)
WARNING: multiple messages have this Message-ID (diff)
From: "Longpeng (Mike)" <longpeng2@huawei.com> To: Paolo Bonzini <pbonzini@redhat.com>, Alex Williamson <alex.williamson@redhat.com> Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org, linux-kernel@vger.kernel.org, Gonglei <arei.gonglei@huawei.com>, Huangzhichao <huangzhichao@huawei.com>, "Longpeng\(Mike\)" <longpeng.mike@gmail.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)
next prev parent reply other threads:[~2019-12-02 10:47 UTC|newest] Thread overview: 10+ 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 9:10 ` Longpeng (Mike) 2019-12-02 9:31 ` Paolo Bonzini 2019-12-02 9:31 ` Paolo Bonzini 2019-12-02 9:42 ` Longpeng (Mike) 2019-12-02 9:42 ` Longpeng (Mike) 2019-12-02 10:06 ` Paolo Bonzini 2019-12-02 10:06 ` Paolo Bonzini 2019-12-02 10:47 ` Longpeng (Mike) [this message] 2019-12-02 10:47 ` Longpeng (Mike)
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