Date: Fri, 8 Jan 1999 11:58:42 -0800 From: Steve Watt <steve@Watt.COM> To: Marc Giannoni <marc@versa.eng.comsat.com> Cc: freebsd-stable@FreeBSD.ORG, Mark Murray <mark@grondar.za>, Chris Dillon <cdillon@wolves.k12.mo.us> Subject: Re: IRQ entropy causes panics? Message-ID: <19990108115842.A14261@wattres.Watt.COM> In-Reply-To: <XFMail.990108151224.marc@versa.eng.comsat.com>; from Marc Giannoni on Fri, Jan 08, 1999 at 02:57:26PM -0500 References: <19990108094015.A10590@wattres.Watt.COM> <XFMail.990108151224.marc@versa.eng.comsat.com>
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On Fri, Jan 08, 1999 at 02:57:26PM -0500, Marc Giannoni wrote:
>
> Hooking interrupts is a "real time" issue. There is no way an OS, however
> will written, can tolerate flagrant additions to it's interrupt latencies.
> ("j. random sysadmin run-amok")
I'm extremely well versed in the issues -- I work in the kernel engineering
group at Lynx Real Time Systems. There are ways to minimize real-time
impacts ("it's just code"), and there are ways to prevent reentrant
corruption. I haven't looked at the IRQ entropy code yet, so I'm
not in a position to say if it _could_ be improved.
> I'm suprised that the randomizer code can hook interrupts at all and still
> keep the OS stable. This stuff is some pretty clever code. You are actually
> modifying the kernel's core execution by enabling these hooks!
Is there a microtime() call already made at each interrupt entry? If
not, then yeah, that would be adding a huge latency. Something on
the order of 5uS on a faster ISA system I tested on (when I added similar
microtime()-like stuff to LynxOS). I haven't looked to see if it's gotten
any better with recent chipsets, though -- that's just way too much cost
for us RTOS guys at interrupt time, so I didn't do that.
> Trust me when I say that there probably is no "Underlying Problem".
> It's pretty amazing that this thing even exists.
I'd still contend, from an engineering standpoint, that such things shouldn't
destabilize a system that way. If it's inherently unstable, then say so.
On the gripping hand, yup, it's a *really* nifty idea.
--
Steve Watt KD6GGD PP-ASEL-IA ICBM: 121W 56' 58.1" / 37N 20' 14.2"
Internet: steve @ Watt.COM Whois: SW32
Free time? There's no such thing. It just comes in varying prices...
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