netdev - Re: [PATCH] Return value from schedule()

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Message-ID: <20080904161424.GA23042@elte.hu>
Date:	Thu, 4 Sep 2008 18:14:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-pci@...r.kernel.org,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH] Return value from schedule()


* Matthew Wilcox <matthew@....cx> wrote:

> In some circumstances, you want to wait for an event to happen.  let's 
> assume that it's a hardware event, so you can't just add a notifier of 
> some kind, you have to poll.  Here's an example:
> 
> On Wed, Sep 03, 2008 at 03:57:13PM -0700, Stephen Hemminger wrote:
> >  			return -ETIMEDOUT;
> > -		udelay(10);
> > +		if (signal_pending(current))
> > +			return -EINTR;
> > +		schedule();
> 
> If there's no other task ready to run, schedule() could return in much
> less than 10 microseconds (actually, it could return in much less than
> 10 microseconds even if another task does run, but let's ignore that case).
> 
> If schedule() returned whether or not it had scheduled another task, we
> could do something like:
> 
> 		if (!schedule())
> 			udelay(10);

hm, i'm not really sure - this really just seems to be a higher prio 
variant of yield() combined with some weird code. Do we really want to 
promote such arguably broken behavior? If there's any chance of any 
polling to take a material amount of CPU time it should be event driven 
to begin with.

	Ingo
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