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

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Date:	Mon, 26 Jul 2010 18:50:15 +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 05:34:44PM +0200, Tejun Heo wrote:
> Hello,
> 
> On 07/26/2010 05:25 PM, Michael S. Tsirkin wrote:
> > BTW, kthread_worker would benefit from the optimization I implemented
> > here as well.
> 
> Hmmm... I'm not quite sure whether it's an optimization.  I thought
> the patch was due to feeling uncomfortable about using barriers?

Oh yes. But getting rid of barriers is what motivated me originally.

>  Is it an optimization?
> 
> Thanks.

Yes, sure. This removes atomic read and 2 barrier operations on data path.  And
it does not add any new synchronization: instead, we reuse the lock that we
take anyway.  The relevant part is:


+               if (work) {
+                       __set_current_state(TASK_RUNNING);
+                       work->fn(work);
+               } else
+                       schedule();

-       if (work) {
-               __set_current_state(TASK_RUNNING);
-               work->fn(work);
-               smp_wmb();      /* wmb worker-b0 paired with flush-b1 */
-               work->done_seq = work->queue_seq;
-               smp_mb();       /* mb worker-b1 paired with flush-b0 */
-               if (atomic_read(&work->flushing))
-                       wake_up_all(&work->done);
-       } else
-               schedule();
-
-       goto repeat;

Is there a git tree with kthread_worker applied?
I might do this just for fun ...


> -- 
> tejun
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