linux-kernel - Re: Considerations on sched APIs under RT patch

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Date:	Wed, 21 Apr 2010 22:58:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Primiano Tucci <p.tucci@...il.com>
Cc:	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	tglx <tglx@...utronix.de>
Subject: Re: Considerations on sched APIs under RT patch

On Wed, 2010-04-21 at 22:38 +0200, Primiano Tucci wrote:
> > No, any syscall can end up blocking/scheduling there are no exceptions.
> > But blocking doesn't mean its non-deterministic, esp. when coupled with
> > things like PI.
> >
> > But you do have to treat system resources as such, that is they can (and
> > will) create cross-cpu dependencies, if you do not take that into
> > account you will of course be surprised.
> >
> I actually don't understand why do you recall PI so frequently, it
> seems to be the unique point of interest.

PI keeps preemptible locks working in a RT environment. Non-preemptible
or preemptible+PI are both valid RT constructs that can be analyzed 

> Actually I take care about not sharing cross-cpu resources, but I
> cannot take care of what the kernel should do.

An SMP kernel must be treated as a cross-cpu resource. There's just no
way around that. For instance, Unix allows two processes on different
cpus to invoke sched_setscheduler/sched_setaffinity or any number of
system calls on the same target process. Filesystems are shared etc..

> In my viewpoint is unacceptable that the scheduler apis can led into a
> rescheduling.

They can even lead to pagefaults and disk IO if you're not careful.

I'm not sure if there are blocking locks left thereabout, but spinlocks
or rt_mutex, both create cross-cpu dependencies that need to be
analyzed, !preempt isn't magic in any way.

> It voids any form of process control.
> If I lose the control while controlling other processes, Quis
> custodiet ipsos custodes?
> 
> P.S. It actually does not happen in other RTOSes, e.g., VxWorks SMP

I don't know any of those, but its impossible to migrate tasks from one
cpu to another without creating cross-cpu dependencies.

Whether locks are preemptible or not doesn't make them any less
analyzable, if you use system-calls in your RT program, their
implementation needs to be considered.

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