linux-kernel - Re: vfio_pin_map_dma cause synchronize

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Message-ID: <42c907fd-6c09-fbb6-d166-60e6827edff5@redhat.com>
Date:   Mon, 2 Dec 2019 11:06:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Longpeng (Mike)" <longpeng2@...wei.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     qemu-devel@...gnu.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Longpeng(Mike)" <longpeng.mike@...il.com>,
        Gonglei <arei.gonglei@...wei.com>,
        Huangzhichao <huangzhichao@...wei.com>
Subject: Re: vfio_pin_map_dma cause synchronize_sched wait too long

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@...ux.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@...zon.de>
    Reported-by: David Woodhouse <dwmw2@...radead.org>
    Suggested-by: Peter Zijlstra <peterz@...radead.org>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>


Thanks,

Paolo