linux-kernel - Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ

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Date:	Wed, 28 Oct 2015 19:28:57 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Yunhong Jiang <yunhong.jiang@...ux.intel.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ



On 28/10/2015 18:50, Yunhong Jiang wrote:
> > No, I don't think you can use raw_spinlock there.  The problem is not
> > just eventfd_signal, it is especially wake_up_locked_poll.  You cannot
> > convert the whole workqueue infrastructure to use raw_spinlock.
> 
> You mean the waitqueue, instead of workqueue, right?

Yes.

> One choice is to change 
> the eventfd to use simple wait queue, which is raw_spinlock. But use simple 
> waitqueue on eventfd may in fact impact real time latency if not in this 
> scenario.

Userspace can put an arbitrary amount of tasks on the work queue, so
it's not possible to use a simple wait queue.  It would also touch
multiple subsystems, so it's much better to bypass the eventfd completely.

Paolo
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