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. Did I get that right? Would be exciting to run the rcutorture test once Paul has it available to reproduce this problem. thanks, - Joel
