Re: Priority Inheritance Test (Real-Time Preemption)

[Ingo Molnar <mingo@elte.hu>]

* Bill Huey <bhuey@lnxw.com> wrote:

> [...] There might be places where, if algorithmically bounded somehow,
> reverting some of the heavy hammered sleeping locks back to spinlocks
> would make the system faster and more controlled. rtc_lock possibly
> could be one of those places and other places that are as heavily as
> used as that.

in the -RT patchset one of the reasons why i've gone for the completely
preemptible variant is to trigger all priority inversion problems
outright. In the first variant they didnt really trigger - but they were
present. Once the locks were almost all preemptible, PI problems
surfaced in a big way - causing people to report them and forcing me to
fix them :-)

There are lots of critical sections in Linux and we cannot design around
them - so if the goal is hard-RT properties and latencies then priority
inversion is a problem that has to be solved. Later on we could easily
revert some of the hw-related spinlocks to raw spinlocks, and/or the
known-O(1) critical sections as well.

the paper cited is not very persuasive to me though. It lists problems
of an incomplete/incorrect PI implementation, and comes to the IMO false
(and unrelated) conclusion that somehow PI-handling is not desired.
Obviously PI makes only sense if it's implemented correctly. I think i
managed to fix the problems Esben's testsuite uncovered, in the current
-RT patch. Anyway, this implementation is also special in that it relies
on correct SMP locking of Linux:

> Turning this into a "priority inheritance world" is just going to turn
> this project into the FreeBSD SMP project [...]

i dont have any intentions to turn Linux into a 'priority inheritance
world'. PI handling is only a property of the PREEMPT_RT feature
intended for the most latency-sensitive applications - the main and
primary critical-section model of Linux is and should still be a healthy
mix of spinlocks and mutexes. Having only mutexes (or only spinlocks) is
an extreme that _does_ hurt the common case. PREEMPT_RT 'only' lives on
the back of SMP-Linux.

	Ingo