linux-kernel - Re: [linux-usb-devel] [4/4] 2.6.23-rc3: known regressions

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Date:	Mon, 20 Aug 2007 23:45:23 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	David Brownell <david-b@...bell.net>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	linux-usb-devel@...ts.sourceforge.net, Greg KH <gregkh@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Stuart_Hayes@...l.com" <Stuart_Hayes@...l.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Exner <dex@...gonslave.de>
Subject: Re: [linux-usb-devel] [4/4] 2.6.23-rc3: known regressions



On Mon, 20 Aug 2007, Arjan van de Ven wrote:

> 
> On Mon, 2007-08-20 at 23:26 -0700, Linus Torvalds wrote:
> > 
> > On Mon, 20 Aug 2007, Arjan van de Ven wrote:
> > > 
> > > untested patch to add this to cpufreq; this is probably a good idea in
> > > general even if using the latency framework doesn't end up being used
> > > for fixing this regression...
> > > 
> > > 
> > > --- linux-2.6.23-rc2/drivers/cpufreq/cpufreq.c.org	2007-08-20 22:58:32.000000000 -0700
> > > +++ linux-2.6.23-rc2/drivers/cpufreq/cpufreq.c	2007-08-20 23:02:21.000000000 -0700
> > > @@ -1604,6 +1604,12 @@ static int __cpufreq_set_policy(struct c
> > >  	if (ret)
> > >  		goto error_out;
> > >  
> > > +
> > > +	if (system_latency_constraint() < policy->cpuinfo.transition_latency) {
> > 
> > That looks broken. "system_latency_constraint()" is in us, but 
> > transition_latency is in ns, afaik.
> > 
> > But adding a "/ 1000" to turn the ns into us, and it migth even work.
> 
> 
> eh woops yes indeed.
> Shows me for not testing; I'll do that tomorrow when I'm more awake

Side note: I think we migth want to also have some way of telling the user 
*why* we're not doing frequency changes. Maybe as simple as a rate-limited 
printk() or something.

Otherwise, we'll easily be in a situation where some poor sod ends up 
running constantly at lowest frequency, and no way of even seeing why. 
Which sounds like a debugging nightmare.

If the kernel spits out the occasional warning about the latency 
violation, at least we get notified about there being potential problems.

		Linus
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