linux-kernel - Re: Busy-waiting with interrupts disabled

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Date:	Mon, 12 Dec 2011 17:47:41 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	USB list <linux-usb@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Busy-waiting with interrupts disabled

On Mon, 2011-12-12 at 17:12 -0500, Alan Stern wrote:
> How long is it okay to busy-wait with interrupts disabled?  Are there 
> any clear-cut guidelines?
> 
> My feeling is that for ordinary desktop use, 1-2 ms should be about the
> limit, but other people may feel differently.  (There's one spot in
> ehci-hcd where the delay can last up to 250 ms, which does seem rather

"250 ms", holy freaking OUCH! Batman!


> excessive.  Fortunately it never takes that long unless the hardware is
> broken.)
> 
> No doubt the RT people would say the upper bound should be on the order
> of a few microseconds or less.  I'd be happy to change ehci-hcd, which
> has several delays in the 1-2 ms range -- but they tend to be nested
> inside routines that are called within the scope of spinlock_irq, which
> means a fair amount of rewriting would be needed.
> 
> Any thoughts or recommendations?

Note that in -rt, spinlocks are really mutexes (unless they are
raw_spinlocks, which you shouldn't do). Thus, if your spin is with
spinlock_irq, it wont affect -rt, as it will not have interrupts
disabled.

But I could imagine that such a long latency could manifest itself into
something nasty (lots of interrupts queued up and such) that normal
desktop users might see a hiccup or two.

-- Steve


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