[patch] voluntary-preempt-2.6.8-rc2-H9

Ingo Molnar
2004-07-22T10:05:59Z

* Scott Wood <scott@timesys.com> wrote:

> > trying to make softirqs preemptible surely wont fly for 2.6 and it will
> > also overly complicate the softirq model. What's so terminally wrong
> > about adding preemption checks to the softirq paths? It should solve the
> > preemption problem for good. The unbound softirq paths are well-known
> > (mostly in the networking code) and already have preemption-alike
> > checks.
>
> If every such loop in every softirq is taken care of, that would work
> (though only until someone adds a new softirq that forgets to check
> for preemption). I don't see any such checks in either the transmit
> or receive network softirqs in vanilla 2.6.7, though (are they in a
> patch, or am I overlooking them?), much less in each individual
> driver. There are checks for excessive work (where "excessive" is not
> well defined in terms of actual time), but none for need_resched()
> except in a few isolated places.

i've added an infrastructure for easy softirq lock-break and preemption
to the -H9 version of the voluntary-preempt patch:

http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.8-rc2-H9

in -H9 i've fixed the most important softirq latency sources:

- the RCU code

this one is new and is really bad - it can literally execute tens of
thousands of d_callback() functions within rcu_do_batch() causing
millisecs of delays - e.g. triggered by the 'du /' test on a box with
enough RAM. It affects UP just as much as SMP, in both preempt and
non-preempt mode as well.

- the timer code

no real latencies in practice but in theory if enough timers are set
to fire in the same jiffy it could be easily unbound.

- net TX/RX code

being the worst offender this had some throttling code already but it
didnt listen to resched requests. It does now.

it is really easy to do lock-break of softirqs, one only has to find a
place where it's safe to enable softirq processing and do a
cond_resched_softirq() call.

Ingo