linux-kernel - Re: [patch 4/5] net: hip04: Make tx coalesce timer actually work

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Date:	Mon, 13 Apr 2015 23:42:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arnd Bergmann <arnd@...db.de>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	dingtianhong <dingtianhong@...wei.com>,
	Zhangfei Gao <zhangfei.gao@...aro.org>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	netdev@...r.kernel.org
Subject: Re: [patch 4/5] net: hip04: Make tx coalesce timer actually work

On Mon, 13 Apr 2015, Arnd Bergmann wrote:

> On Monday 13 April 2015 21:02:23 Thomas Gleixner wrote:
> > The code sets the expiry value of the timer to a relative value and
> > starts it with hrtimer_start_expires. That's fine, but that only works
> > once. The timer is started in relative mode, so the expiry value gets
> > overwritten with the absolut expiry time (now + expiry).
> > 
> > So once the timer expired, a new call to hrtimer_start_expires results
> > in an immidiately expired timer, because the expiry value is
> > already in the past.
> > 
> > Use the proper mechanisms to (re)start the timer in the intended way.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: dingtianhong <dingtianhong@...wei.com>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Zhangfei Gao <zhangfei.gao@...aro.org>
> > Cc: Dan Carpenter <dan.carpenter@...cle.com>
> > Cc: netdev@...r.kernel.org
> 
> Thanks a lot for the fix. The mistake was clearly mine, as I had sent
> a patch to introduce the tx coalesce timer without access to hardware
> or a way to test that what I did was correct.
> 
> There are other known problems in the version of the driver that got
> merged, and I believe that someone is now looking at them.
> 
> What I think we really want here is a way for user space to configure
> both the minimum and maximum coalesce timer separately rather than
> assuming half the time is what we want.
> 
> 	Arnd
> 
> > @@ -413,6 +413,15 @@ out:
> >  	return count;
> >  }
> >  
> > +static void hip04_start_tx_timer(struct hip04_priv *priv)
> > +{
> > +	ktime_t t;
> > +
> > +	/* allow timer to fire after half the time at the earliest */
> > +	t = ktime_set(0, priv->tx_coalesce_usecs * NSEC_PER_USEC / 2);
> > +	hrtimer_start(&priv->tx_coalesce_timer, t, HRTIMER_MODE_REL);
> > +}
> 
> Question: this looks to me like it sets both the minimum and maximum
> time to priv->tx_coalesce_usecs/2, when the intention was to set
> the minimum to priv->tx_coalesce_usecs/2 and the maximum to
> priv->tx_coalesce_usecs. Am I missing something subtle here, or did
> you just misread my original intention from the botched code?

Yes, I missed that. Simple fix for this is:

  unsigned long t_ns = priv->tx_coalesce_usecs * NSEC_PER_USEC / 2;
  
  hrtimer_start_range_ns(&priv->tx_coalesce_timer, ns_to_ktime(t_ns),
			 t_ns, HRTIMER_MODE_REL);

Thanks,

	tglx
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