netdev - Re: [PATCH UPDATED 1/3] vhost: replace vhost

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Date:	Tue, 27 Jul 2010 22:19:11 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Sridhar Samudrala <sri@...ibm.com>,
	netdev <netdev@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitri Vorobiev <dmitri.vorobiev@...ial.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH UPDATED 1/3] vhost: replace vhost_workqueue with
 per-vhost kthread

On Mon, Jul 26, 2010 at 09:31:58PM +0200, Tejun Heo wrote:
> Hello,
> 
> On 07/26/2010 09:14 PM, Tejun Heo wrote:
> > On 07/26/2010 06:51 PM, Michael S. Tsirkin wrote:
> >> I noticed that with vhost, flush_work was getting the worker
> >> pointer as well. Can we live with this API change?
> > 
> > Yeah, the flushing mechanism wouldn't work reliably if the work is
> > queued to a different worker without flushing, so yeah passing in
> > @worker might actually be better.
> 
> Thinking a bit more about it, it kind of sucks that queueing to
> another worker from worker->func() breaks flush.  Maybe the right
> thing to do there is using atomic_t for done_seq?

I don't believe it will help: we might have:

worker1 runs work
work requeues itself queued index = 1
worker1 reads queued index = 1
worker2 runs work
work requeues itself queued index = 2
worker2 runs work
worker2 reads queued index = 2
worker2 writes done index = 2
worker1 writes done index = 1

As you see, done index got moved back.



>  It pays a bit more
> overhead but maybe that's justifiable to keep the API saner?  It would
> be great if it can be fixed somehow even if it means that the work has
> to be separately flushed for each worker it has been on before being
> destroyed.
> 
> Or, if flushing has to be associated with a specific worker anyway,
> maybe it would be better to move the sequence counter to
> kthread_worker and do it similarly with the original workqueue so that
> work can be destroyed once execution starts?  Then, it can at least
> remain semantically identical to the original workqueue.
> 
> Thanks.
> 
> -- 
> tejun
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