linux-kernel - Re: serial flow control appears broken

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Date:	Mon, 30 Jul 2007 10:59:33 +0100
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Robert Hancock <hancockr@...w.ca>,
	Lee Howard <faxguy@...ardsilvan.com>,
	linux-serial@...r.kernel.org, tytso@....edu,
	linux-kernel@...r.kernel.org
Subject: Re: serial flow control appears broken

On Mon, Jul 30, 2007 at 10:45:19AM +0100, Maciej W. Rozycki wrote:
> On Sat, 28 Jul 2007, Russell King wrote:
> 
> > Essentially, any complex interrupt handler (such as an IDE interrupt
> > doing a multi-sector PIO transfer _in interrupt context_) can cause this
> > kind of starvation.  That's why Linux 1.x had bottom halves - so that
> > the time consuming work could be moved out of the interrupt handler,
> > thereby causing minimal the blockage of other interrupts.
> > 
> > Unfortunately, that kind of design has been long since forgotten.
> > Apparantly modern machines are fast enough that it doesn't have to be
> > worried about anymore...  Or are they?
> 
>  I would guess it is not that the machines are fast enough, but that this 
> two-level processing makes things more complicated.  Enough that most 
> people would not bother digging into it unless really forced.  Only 
> occasional latency problems are probably not enough of a force.

It's a shame we don't have a way to measure IRQ latency - it would be
very useful to flag up problems.

I think the best we could do is to arrange for the timer interrupt to
complain if it's delayed by more than 1ms or so - but some architectures
already run their timers with IRQF_DISABLED as a work around some of
the latency issues.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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