21 Jul
2004
21 Jul
'04
4:45 p.m.
On Wed, Jul 21, 2004 at 10:22:18AM +0200, Ingo Molnar wrote:
we already 'daemonize' softirqs in the stock
kernel if the load is high
enough. (this is what ksoftirqd does) So the only question is a tunable
to make this deferring of softirq load mandatory. Yarroll's patch is
quite complex, i dont think that is necessary.
What aspects of it do you find unnecessary? The second thread is
needed to maintain the current high/low priority semantics (without
that, you'll either starve regular tasks with a lot of softirqs, or
starve softirqs with a busy userspace, depending on how you set the
priority of the softirq thread).
It also has at least one
conceptual problem, the NOP-ing of local_bh_disable/enable in case of
CONFIG_SOFTIRQ_THREADS is clearly wrong. Yarroll?
Why is it "clearly wrong"? As far as I can tell, the only legitimate
use of it currently is to protect against deadlock (as in
spin_lock_bh()), which is not an issue if all softirqs run from a
thread. Ksoftirqd already ignores such disabling (unless I'm missing
something?), so code that uses it to synchronize with a softirq is
already broken.
There's also the possibility of code relying on it also being
preempt_disable(); if that's happening, it could be left alone
(though IMHO it'd be better if such code made its dependence on such
behavior explicit), with preempt_disable() being the only real
effect.
I've added a very simple solution to daemonize
softirqs similar to
Yarroll's patch to the -H5 version of voluntary-preempt:
BTW, it was my patch; Yarroll only submitted it to the list (as he
stated at the time).
-Scott