On Tue, Mar 17, 2015 at 05:56:21PM +0100, Peter Zijlstra wrote: > On Tue, Mar 17, 2015 at 03:08:46PM +0100, Peter Zijlstra wrote: > > On Tue, Mar 17, 2015 at 04:36:48AM -0700, Paul E. McKenney wrote: > > > On Tue, Mar 17, 2015 at 09:18:07AM +0100, Peter Zijlstra wrote: > > > > On Mon, Mar 16, 2015 at 11:37:45AM -0700, Paul E. McKenney wrote: > > > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > > > > > > > > > > RCU ignores offlined CPUs, so they cannot safely run RCU read-side code. > > > > > (They -can- use SRCU, but not RCU.) This means that any use of RCU > > > > > during or after the call to arch_cpu_idle_dead(). Unfortunately, > > > > > commit 2ed53c0d6cc99 added a complete() call, which will contain RCU > > > > > read-side critical sections if there is a task waiting to be awakened. > > > > > > > > Got a little more detail there? > > > > > > Quite possibly. But exactly what sort of detail are you looking for? > > > > What exact RCU usage you ran into that was problematic. It seems to > > imply that calling complete() -- from a dead cpu -- which ends up in > > try_to_wake_up() was the problem? > > Hmm, I'm thinking its select_task_rq_*(). And yes, 'fixing' this in the > wake-up path will penalize everybody for the benefit of the very rare > case someone is doing a hotplug. > > So yeah, maybe this is the best solution.. Ulgy though :/ Ugly indeed! I end up doing a polling loop for the generic code. For the first round, I updated only architectures that were calling complete(). If that goes well, I will probably update some of the other architecture as a code-consolidation measure. Some architectures have special hardware and firmware hooks, for example, s390 uses a special instruction to do the wakeup directly. Those will of course continue doing their own thing. The ARM guys are trying to do something specific to their hardware, but I have not heard from them lately. I should ping them... Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
