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Date: Thu, 28 Nov 2013 13:23:48 +0200
From: Gleb Natapov <gleb@minantech.com>
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Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to
 update vm irq routing table
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Avi Kivity <avi@cloudius-systems.com>, "Huangweidong (C)" <weidong.huang@huawei.com>, Gleb Natapov <gleb@redhat.com>, KVM <kvm@vger.kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>, "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Zanghongyong <zanghongyong@huawei.com>, Avi Kivity <avi.kivity@gmail.com>, "Jinxin (F)" <jinxin712@huawei.com>

On Thu, Nov 28, 2013 at 12:10:40PM +0100, Paolo Bonzini wrote:
> Il 28/11/2013 12:09, Gleb Natapov ha scritto:
> > > - if there are no callbacks, but there are readers, synchronize_srcu
> > > busy-loops for some time checking if the readers complete.  After a
> > > while (20 us for synchronize_srcu, 120 us for
> > > synchronize_srcu_expedited) it gives up and starts using a workqueue to
> > > poll every millisecond.  This should never happen unless
> > 
> > Unless what ? :) Unless reader is scheduled out?
> 
> Yes.  Or unless my brain is scheduled out in the middle of a sentence.
> 
So we will have to disable preemption in a reader to prevent big latencies for
a writer, no?

--
			Gleb.