Re: [2.4.17/18pre] VM and swap - it's really unusable

Andrew Morton
2002-01-08T22:27:17Z

Daniel Phillips wrote:
>
> On January 8, 2002 08:47 pm, Andrew Morton wrote:
> > Daniel Phillips wrote:
> > > What a preemptible kernel can do that a non-preemptible kernel can't is:
> > > reschedule exactly as often as necessary, instead of having lots of extra
> > > schedule points inserted all over the place, firing when *they* think the
> > > time is right, which may well be earlier than necessary.
> >
> > Nope. `if (current->need_resched)' -> the time is right (beyond right,
> > actually).
>
> Oops, sorry, right.
>
> The preemptible kernel can reschedule, on average, sooner than the
> scheduling-point kernel, which has to wait for a scheduling point to roll
> around.
>

Yes. It can also fix problematic areas which my testing
didn't cover.

Incidentally, there's the SMP problem. Suppose we
have the code:

lock_kernel();
for (lots) {
do(something sucky);
if (current->need_resched)
schedule();
}
unlock_kernel();

This works fine on UP, but not on SMP. The scenario:

- CPU A runs this loop.

- CPU B is spinning on the lock.

- Interrupt occurs, kernel elects to run RT task on CPU B.
CPU A doesn't have need_resched set, and just keeps
on going. CPU B is stuck spinning on the lock.

This is only an issue for the low-latency patch - all the
other approaches still have sufficiently bad worse-case that
this scenario isn't worth worrying about.

I toyed with creating spin_lock_while_polling_resched(),
but ended up changing the scheduler to set need_resched
against _all_ CPUs if an RT task is being woken (yes, yuk).

-