Messages in this thread |  | | Date | Tue, 08 Jan 2002 14:21:11 -0800 | From | Andrew Morton <> | Subject | Re: [2.4.17/18pre] VM and swap - it's really unusable |
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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).
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