netdev - Re: [PATCH] vhost: get 2% performance improved by reducing spin_lock race in vhost_work

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Date:	Tue, 21 May 2013 02:40:42 +0000
From:	Qinchuanyu <qinchuanyu@...wei.com>
To:	"mst@...hat.com" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>
CC:	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"(kvm@...r.kernel.org)" <kvm@...r.kernel.org>,
	"(netdev@...r.kernel.org)" <netdev@...r.kernel.org>,
	Heguansen <heguansen@...wei.com>
Subject: Re: [PATCH] vhost: get 2% performance improved by reducing
 spin_lock race in vhost_work_queue

From: Chuanyu Qin <qinchuanyu@...wei.com>
Subject: [PATCH] get 2% or more performance improved by reducing spin_lock race in vhost_work_queue

the wake_up_process func is included by spin_lock/unlock in vhost_work_queue, 
but it could be done outside the spin_lock. 
I have test it with kernel 3.0.27 and guest suse11-sp2 using iperf, the num as below.
                 orignal                   modified
thread_num  tp(Gbps)   vhost(%)  |  tp(Gbps)     vhost(%)
1           9.59         28.82   |      9.59        27.49
8            9.61        32.92   |      9.62        26.77
64            9.58        46.48  |     9.55        38.99
256            9.6        63.7   |      9.6         52.59

Signed-off-by: Chuanyu Qin <qinchuanyu@...wei.com>
---
 drivers/vhost/vhost.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 94dbd25..8bee109 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -146,9 +146,10 @@ static inline void vhost_work_queue(struct vhost_dev *dev,
 	if (list_empty(&work->node)) {
 		list_add_tail(&work->node, &dev->work_list);
 		work->queue_seq++;
+		spin_unlock_irqrestore(&dev->work_lock, flags);
 		wake_up_process(dev->worker);
-	}
-	spin_unlock_irqrestore(&dev->work_lock, flags);
+	} else
+		spin_unlock_irqrestore(&dev->work_lock, flags);
 }
 
 void vhost_poll_queue(struct vhost_poll *poll)
-- 
1.7.3.1.msysgit.0


> On 05/20/2013 12:22 PM, Qinchuanyu wrote:
> > The patch below is base on
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-
> next.git/tree/drivers/vhost/vhost.c?id=refs/tags/next-20130517
> >
> > Signed-off-by: Chuanyu Qin <qinchuanyu@...wei.com>
> > --- a/drivers/vhost/vhost.c     2013-05-20 11:47:05.000000000 +0800
> > +++ b/drivers/vhost/vhost.c     2013-05-20 11:48:24.000000000 +0800
> > @@ -154,9 +154,10 @@
> >         if (list_empty(&work->node)) {
> >                 list_add_tail(&work->node, &dev->work_list);
> >                 work->queue_seq++;
> > +               spin_unlock_irqrestore(&dev->work_lock, flags);
> >                 wake_up_process(dev->worker);
> > -       }
> > -       spin_unlock_irqrestore(&dev->work_lock, flags);
> > +       } else
> > +               spin_unlock_irqrestore(&dev->work_lock, flags);
> >  }
> >
> >  void vhost_poll_queue(struct vhost_poll *poll)
> >
> > I did the test by using iperf in 10G environment, the test num as
> below:
> >                  orignal                   modified
> > thread_num  tp(Gbps)   vhost(%)  |  tp(Gbps)     vhost(%)
> > 1           9.59         28.82   |      9.59        27.49
> > 8            9.61        32.92   |      9.62        26.77
> > 64            9.58        46.48  |     9.55        38.99
> > 256            9.6        63.7   |      9.6         52.59
> >
> > The cost of vhost reduced while the throughput is almost unchanged.
> 
> Thanks, and please generate a formal patch based on
> Documentation/SubmittingPatches (put the description and perf numbers
> in the commit log). Then resubmit it to let the maintainer apply it.