On Thu, Jul 18, 2019 at 02:34:19PM -0700, Paul E. McKenney wrote: > On Thu, Jul 18, 2019 at 12:14:22PM -0400, Joel Fernandes wrote: > > Trimming the list a bit to keep my noise level low, > > > > On Sat, Jul 13, 2019 at 1:41 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote: > > [snip] > > > > It still feels like you guys are hyperfocusing on this one particular > > > > > knob. I instead need you to look at the interrelating knobs as a group. > > > > > > > > Thanks for the hints, we'll do that. > > > > > > > > > On the debugging side, suppose someone gives you an RCU bug report. > > > > > What information will you need? How can you best get that information > > > > > without excessive numbers of over-and-back interactions with the guy > > > > > reporting the bug? As part of this last question, what information is > > > > > normally supplied with the bug? Alternatively, what information are > > > > > bug reporters normally expected to provide when asked? > > > > > > > > I suppose I could dig out some of our Android bug reports of the past where > > > > there were RCU issues but if there's any fires you are currently fighting do > > > > send it our way as debugging homework ;-) > > > > > > Suppose that you were getting RCU CPU stall > > > warnings featuring multi_cpu_stop() called from cpu_stopper_thread(). > > > Of course, this really means that some other CPU/task is holding up > > > multi_cpu_stop() without also blocking the current grace period. > > > > > > > So I took a shot at this trying to learn how CPU stoppers work in > > relation to this problem. > > > > I am assuming here say CPU X has entered MULTI_STOP_DISABLE_IRQ state > > in multi_cpu_stop() but another CPU Y has not yet entered this state. > > So CPU X is stalling RCU but it is really because of CPU Y. Now in the > > problem statement, you mentioned CPU Y is not holding up the grace > > period, which means Y doesn't have any of IRQ, BH or preemption > > disabled ; but is still somehow stalling RCU indirectly by troubling > > X. > > > > This can only happen if : > > - CPU Y has a thread executing on it that is higher priority than CPU > > X's stopper thread which prevents it from getting scheduled. - but the > > CPU stopper thread (migration/..) is highest priority RT so this would > > be some kind of an odd scheduler bug. > > - There is a bug in the CPU stopper machinery itself preventing it > > from scheduling the stopper on Y. Even though Y is not holding up the > > grace period. > > - CPU Y might have already passed through its quiescent state for > the current grace period, then disabled IRQs indefinitely. > Now, CPU Y would block a later grace period, but CPU X is > preventing the current grace period from ending, so no such > later grace period can start. Ah totally possible, yes! thanks, - Joel
