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 -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
