On Thu, May 19, 2022 at 05:05:04PM -0400, Qian Cai wrote: > On Thu, May 19, 2022 at 12:15:24PM -0700, Paul E. McKenney wrote: > > Is the task doing offline_pages()->synchronize_rcu() doing this > > repeatedly? Or is there a stalled RCU grace period? (From what > > I can see, offline_pages() is not doing huge numbers of calls to > > synchronize_rcu() in any of its loops, but I freely admit that I do not > > know this code.) > > Yes, we are running into an endless loop in isolate_single_pageblock(). > There was a similar issue happened not long ago, so I am wondering if we > did not solve it entirely then. Anyway, I will continue the thread over > there. > > https://lore.kernel.org/all/YoavU%2F+NfQIzQiDF@qian/ I do know that feeling. > > Or is it possible that reverting those three patches simply decreases > > the probability of failure, rather than eliminating the failure? > > Such a decrease could be due to many things, for example, changes to > > offsets and sizes of data structures. > > Entirely possible. Sorry for the false alarm. Not a problem! > > Do you ever see RCU CPU stall warnings? > > No. OK, then perhaps a sequence of offline_pages() calls. Hmmm... The percpu_up_write() function sets ->block to zero before awakening waiters. Given wakeup latencies, might this allow an only somewhat unfortunate sequence of events to allow offline_pages() to starve readers? Or is there something I am missing that prevents this from happening? Thanx, Paul
