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From: Roger Larsson <roger.larsson@norran.net>
To: "Richard B. Johnson" <root@chaos.analogic.com>,
        Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: SMP spin-locks
Date: Thu, 14 Jun 2001 22:42:03 +0200
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Hi,

Wait a minute...

Spinlocks on a embedded system? Is it _really_ SMP?

What kind of performance problem do you have?
My guess, since you are mentioning spin locks, is that you are
having a latency problem - RT process does not execute/start
quickly enough?

If that is the case you should look at Andrew Mortons low latency
patches.
 http://www.uow.edu.au/~andrewm/linux/schedlat.html

/RogerL

On Thursday 14 June 2001 19:26, Richard B. Johnson wrote:
> I __finally__ got back on "the list". They finally fixed the
> company firewall!
>
> During my absence, I had the chance to look at some SMP code
> because of a performance problem (a few microseconds out of
> spec on a 130 MHz embedded system) and I have a question about
> the current spin-locks.
>
> Spin-locks now transfer control to the .text.lock segment.
> This is a separate segment that can be at an offset that
> is far away from the currently executing code. That may
> cause the cache to be reloaded. Further, each spin-lock
> invocation generates separate code within that segment.
>
> Question 1: Why?
>
> Question 2: What is the purpose of the code sequence, "repz nop"
> generated by the spin-lock code? Is this a processor BUG work-around?
> `as` doesn't "like" this sequence and, Intel doesn't seem to
> document it.
>
>
> Cheers,
> Dick Johnson
>
> Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).
>
> "Memory is like gasoline. You use it up when you are running. Of
> course you get it all back when you reboot..."; Actual explanation
> obtained from the Micro$oft help desk.
>
>
> -
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-- 
Roger Larsson
Skellefteċ
Sweden