linux-kernel - Re: A better interface, perhaps: a timed signal flag

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Date:	Fri, 28 Jul 2006 12:44:49 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Theodore Tso <tytso@....edu>, Neil Horman <nhorman@...driver.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Dave Airlie <airlied@...il.com>, linux-kernel@...r.kernel.org,
	a.zummo@...ertech.it, jg@...edesktop.org
Subject: Re: A better interface, perhaps: a timed signal flag

On Fri, 2006-07-28 at 17:41 +0100, Alan Cox wrote:
> Ar Gwe, 2006-07-28 am 10:52 -0400, ysgrifennodd Theodore Tso:
> > Good point, and limiting this facility to one such timeout per
> > task_struct seems like a reasonable restriction. 
> 
> Why is this any better than using a thread or signal handler ? From the
> implementation side its certainly horrible - we will be trying to write
> user pages from an IRQ event. Far better to let the existing thread code
> deal with it.
> 

If the user page is special, in that it is really a kernel page mapped
to userspace.  The implementation on making sure it doesn't disappear on
the interrupt isn't that difficult.

But for real-time applications, the signal handling has a huge latency.
Where as what Theodore wants to do is very light weight.  ie. have a
high prio task doing smaller tasks until a specific time that tells it
to stop.  Having a signal, would create the latency on having that task
stop.

These little requests make sense really only in the real-time space.
The normal uses can get by with signals.  But I will say, the normal
uses for computing these days are starting to want the real-time
powers. :)

-- Steve

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