On Fri, Jul 11, 2008 at 02:11:59PM +1000, Nick Piggin wrote:
> On Friday 11 July 2008 04:06, Ingo Molnar wrote:
>
> > i'm wondering why rcutorture didnt trigger it. I do run !HOTPLUG +
> > RCU_PREEMPT kernels and never saw this. Nor did Paul. That aspect is
> > weird.
>
> It basically requires an active rcu reader to be preempted (preferably
> by something doing a lot of call_rcu or other activity ie. the writer
> so it can tick along the different states quickly).
>
> I found just 2 threads (reader and writer) bound to the same CPU would
> trigger it fastest, my reader has quite a long rcu read section.
>
> I'm not sure why rcutorture doesn't trigger for everyone. I'm surprised
> it does not have much longer maximum read delays -- several ms I would
> have thought should be useful to have a crticial section open while the
> rcu engine can run through a number of states...
Hit it in 10 seconds once I actually got HOTPLUG_CPU disabled.
The theory behind the default settings for rcutorture are as follows:
o Having two reader threads for each CPU helps ensure interactions
between those threads.
o The writer is normally going to have to share a CPU with a
reader or two, maybe three. This should force reader-writer
interactions.
o The read-hold time needs to be long enough to ensure interactions
with the writer, but if it is too long, there are too few
rcu_read_lock() and rcu_read_unlock() events to really stress
the read-side processing.
o The four fakewriters ensure interaction between multiple
writers.
To Nick's point, I did use a hacked-up rcutorture with millisecond
read-side delays when debugging preemptable RCU, but I also used stock
rcutorture.
I will give this some thought and see if the defaults should change or
if more knobs are needed.
Thanx, Paul
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