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Re: [Ext-rt-dev] Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

From:  Bill Huey (hui) <bhuey-AT-lnxw.com>
To:  Thomas Gleixner <tglx-AT-linutronix.de>
Subject:  Re: [Ext-rt-dev] Re: [ANNOUNCE] Linux 2.6 Real Time Kernel
Date:  Tue, 12 Oct 2004 14:12:01 -0700
Cc:  dwalker-AT-mvista.com, Ingo Molnar <mingo-AT-elte.hu>, Andrew Morton <akpm-AT-osdl.org>, amakarov-AT-ru.mvista.com, ext-rt-dev-AT-mvista.com, LKML <linux-kernel-AT-vger.kernel.org>

On Tue, Oct 12, 2004 at 09:46:34PM +0200, Thomas Gleixner wrote:
> Both patches (MV & Ingos) have their good bits, but both share the same
> ugliness and are hard to compare and harder to combine. The conversion
> of spin_lock to _spin_lock and substitution of spin_lock by mutexes,
> semaphores or what ever makes it more than hard to keep the code in a
> readable form.
> 
> If there is the tendency to touch the concurrency controls in general
> all over the kernel, then I would suggest a script driven overhaul of
> all concurrency controls like spin_locks, mutexes and semaphores to
> general macros like 
> 
> enter_critical_section(TYPE, &var, &flags, whatever);
> leave_critical_section(TYPE, &var, flags, whatever);

FreeBSD uses these things, but it they create severe pipeline stalls
since they toggle interrupt flags on entry and exit. The current scheme
in Linux with preempt_count use to be a curse when I was working on an
equivalent implementation of there stuff at:

	http://mmlinux.sf.net

It's a project I've been working for a long time and I'm farther than
them in the area of stability and most likely the problem space in general.
They are 7 and I am a single engineer though.

I don't have the latest sources up and I'm going to up load them in a
couple of hours. I've been playing with it for about 2 months, late July,
since it was able to boot reliably and I've felt/measure how a fully
preemptable kernel like this can perform. I'm getting about 4-6 usecs
average latency in the system from interrupt exception frame to the start
of the irq-thread in question. Tons of events were at 2 usecs which I
thought was insane at the time, but a ndelay insert into the path verified
this to be correct.  The majority of the spread was at 5 and 10 usecs,
pushing to about 12 usecs. That's fantastic latency performance and I
was floored when the measurements were validating my preemption ideas
at the time.

> where TYPE might be SPIN_LOCK, SPIN_LOCK_IRQ, MUTEX, PMUTEX or whatever
> we have and come up with in the future.

There's two problems that need to be solved at this moment regarding
this issue. One is long term which should have a clear differentiation
of what is a persistent spinlock across a compile .config context
(choice of preemptable or standard kernels) is useful since it clearly
identifies which devices and low level systems. The other is Ingo's need
to be able to rapidly change mutexes at the drop of a hat. Eventually,
the long term goal will impose on stylistic issues in the Linux kernel
community and papers/documentation will have to be written to describe
these changes across all kernel subsystems and drivers. It's complete
epic flame bait.

In my system, I do exactly what you just outlined. With a three character
"vim" command, I capitalize the entire word, spin_lock -> SPIN_LOCK
repeated with a ".". I choose this convention because capitals standout
broadly in the source code. It's good because having this kind of
visibility can show static/compile time sleep violations that are the
main source of instability, and almost certainly all of the deadlocks
in Monta Vista's current preemption release.

My tree is stable. I was able to hammer this machine for 2-3 days straight
(no networking, that's another major can of worms) with deadlocking
using multipule mass "find / -exec egrep" of some sort that stress both
process creation and all parts of the IO system.

The lock graph changes I made ironically outlined some serious Linux
structural problems as it concerns latency. Through my effort of fixing
all of the sleep violation, I came all of the way back to the start of
the project which is that all major systems have become non-preemptable
again.

That graph that I saw from Lee is consistent with my results in that a
deadlock prone system will have phenomenal latency performance at the
expense of being absolutely incorrect. It's just a flat out broken
system at this point that they've released.

> This could be done in a first step and then it is clearly identifiable
> and it gives us more flexibility to wrap different implementations and
> lets us change particular points in a more clear way.

Yes, I agree, but the convention needs to be standardized.
 
> I would be willing to provide some scripted conversion aid, if there is
> enough interest to that. I started with some test files and the results
> are quite encouraging.

No, all of this can only be manual at this time, either through static
analysis by a compiler, like what Ingo did over the weekend or by hand
with runtime sleep violation checks.

Give me a bit of time to upload those files. I was just given permission
to talk about this openly now. But I can definitely tell you that I had
this running months before Monta Vista's announcement over the weekend.

Full preemption has just heated up in serious way. :) It's going to be
interesting.
 
> Any thoughts ?

bill

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