linux-kernel - Re: [PATCH 3/4] Make net watchdog timers 1 sec jiffy aligned

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Date:	Wed, 30 May 2007 17:10:39 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	David Miller <davem@...emloft.net>
Cc:	kaber@...sh.net, shemminger@...ux-foundation.org,
	akpm@...ux-foundation.org, venkatesh.pallipadi@...el.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 3/4] Make net watchdog timers 1 sec jiffy aligned

On Wed, May 30, 2007 at 12:55:51PM -0700, David Miller wrote:
> From: Patrick McHardy <kaber@...sh.net>
> Date: Wed, 30 May 2007 20:42:32 +0200
> 
> > Stephen Hemminger wrote:
> > >>>Index: linux-2.6.22-rc-mm/net/sched/sch_generic.c
> > >>>===================================================================
> > >>>--- linux-2.6.22-rc-mm.orig/net/sched/sch_generic.c	2007-05-24 11:16:03.000000000 -0700
> > >>>+++ linux-2.6.22-rc-mm/net/sched/sch_generic.c	2007-05-25 15:10:02.000000000 -0700
> > >>>@@ -224,7 +224,8 @@
> > >>> 	if (dev->tx_timeout) {
> > >>> 		if (dev->watchdog_timeo <= 0)
> > >>> 			dev->watchdog_timeo = 5*HZ;
> > >>>-		if (!mod_timer(&dev->watchdog_timer, jiffies + dev->watchdog_timeo))
> > >>>+		if (!mod_timer(&dev->watchdog_timer,
> > >>>+			       round_jiffies(jiffies + dev->watchdog_timeo)))
> > >>> 			dev_hold(dev);
> > >>> 	}
> > >>> }
> > >>
> > >>Please cc netdev on net patches.
> > >>
> > >>Again, I worry that if people set the watchdog timeout to, say, 0.1 seconds
> > >>then they will get one second, which is grossly different.
> > >>
> > >>And if they were to set it to 1.5 seconds, they'd get 2.0 which is pretty
> > >>significant, too.
> > > 
> > > 
> > > Alternatively, we could change to a timer that is pushed forward after each
> > > TX, maybe using hrtimer and hrtimer_forward().  That way the timer would
> > > never run in normal case.
> > 
> > 
> > It seems wasteful to add per-packet overhead for tx timeouts, which
> > should be an exception. Do drivers really care about the exact
> > timeout value? Compared to a packet transmission time its incredibly
> > long anyways ..
> 
> I agree, this change is absolutely rediculious and is just a blind
> cookie-cutter change made without consideration of what the code is
> doing and what it's requirements are.

What are you agreeing with, Dave?

Are you agreeing that "it seems wasteful to add per-packet overhead"?
This patch is not doing that.

A quick grep shows that most things are using multi-second timeouts
here. Of the ones that aren't, a number are using .4s, and many more
aren't even in units of HZ. Makes me wonder if the various boards
using 50ms are being overzealous.

-- 
Mathematics is the supreme nostalgia of our time.
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