Re: RT Mutex patch and tester [PREEMPT_RT]

[Esben Nielsen <simlo@phys.au.dk>]

On Fri, 13 Jan 2006, Bill Huey wrote:

> On Thu, Jan 12, 2006 at 01:54:23PM +0100, Esben Nielsen wrote:
> > turnstiles? What is that?
>
> http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/kern/subr_turnstile.c
>
> Please, read. Now tell me or not if that looks familiar ? :)

Yes, it reminds me of Ingo's first approach to pi locking:
Everything is done under a global spin lock. In Ingo's approach it was the
pi_lock. In turnstiles it is sched_lock, which (without looking at other
code in FreeBSD) locks the whole scheduler.

Although making the code a lot simpler, scalability is ofcourse the main
issue here. But apparently FreeBSD does have a global lock protecting the
scheduler anyway.

Otherwise it looks a lot like the rt_mutex.

Esben


>
> Moving closer an implementation is arguable, but it is something that
> should be considered somewhat since folks in both the Solaris (and
> FreeBSD) communities have given a lot more consideration to these issues.
>
> The stack allocated objects are fine for now. Priority inheritance
> chains should never get long with a fine grained kernel, so the use
> of a stack allocated object and migrating pi-ed waiters should not
> be a major real world issue in Linux yet.
>
> Folks should also consider using an adaptive spin in the __grab_lock() (sp?)
> related loops as a possible way of optimizing away the immediate blocks.
> FreeBSD actually checks the owner of a lock aacross another processor
> to see if it's actively running, "current", and will block or wait if
> it's running or not respectively. It's pretty trivial code, so it's
> not a big issue to implement. This is ignoring the CPU local storage
> issues.
>
> bill
>
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