Re: [linux-audio-dev] Re: [announce] [patch] Voluntary Kernel Preemption Patch

[Ingo Molnar <mingo@elte.hu>]

* Scott Wood <scott@timesys.com> 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).

what high/low semantics do you mean, other than the ordering of softirq
sources? (which is currently implemented via the __do_softirq() loop
first looking at the highest prio softirq.) So splitting up ksoftirqd
into two pieces seems like a separate issue.

> > 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.

local_bh_disable() excludes all softirq processing. This means that such
a section must not be preempted. E.g. the networking layer manipulates
per-CPU lists from such sections, if you remove local_bh_disable() then
from the middle of such a section we could preempt into ksoftirqd which
would break the code.

> 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.

yes, that's how softirqs are used. The patch changes these semantics.

> > 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).

ok - sorry about the misattribution!

	Ingo