[EMAIL PROTECTED] wrote:
> On Fri, Dec 01, 2000 at 02:22:11PM -0700, Gordon McNutt wrote:
> > Looking at some of the examples, it appears that an RT interrupt handler
> > can invoke the RT scheduler (by calling pthread_wakeup_np, sem_post,
> > etc). And it looks like the scheduler is going to swap in a runnable
> > task when this happens.
> >
> > This doesn't seem safe... I'm still thinking about it...
>
> It is.
>
After some thought, I guess I have to agree. However, an interrupt handler
should be written so that if it calls sem_post() (for example), it should
expect that an arbitrary amount of time has passed before sem_post returns.
In fact, the interrupt handler may have been reentered in this interim, since
it won't return from the sem_post until the task it interrupted gets
scheduled to run again. This is fine as long as it's written to expect this
scenario.
>
> >
> > My (limited) knowledge of interrupts & schedulers indicates that the
> > scheduler should just mark the task as ready to run and return. Then,
>
> Return to what?
>
Bad phrasing on my part. The sem_post might mark the target thread as ready
to run and only call the scheduler if it is not in an interrupt context.
> > after the interrupt handling code exits, the scheduler is called to pick
> > the next task to run.
>
> Called from where?
Well, in the x86 code, the 'reschedule' label in entry.S, which can be
reached from the 'ret_from_intr' label in the same file.
>
> There must be some interrupt operation that can force a scheduling
> or else you have only voluntary preemption.
>
Sure. But sem_post doesn't have to start running the new task right there, it
can mark it as ready and return. Rescheduling can occur after the interrupt
(and any other pending interrupts) have been handled. At least from my
(admittedly superficial) examination the Linux scheduler and interrupt
handling seem to work this way.
However, for a real-time system I think that the RT scheduler probably uses a
better technique... I'll think about it some more over the weekend.
Thanks,
--Gordon
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