On Wed, 6 May 2020 07:41:51 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > On Wed, May 06, 2020 at 02:59:26PM +0200, SeongJae Park wrote: > > TL; DR: It was not kernel's fault, but the benchmark program. > > > > So, the problem is reproducible using the lebench[1] only. I carefully read > > it's code again. > > > > Before running the problem occurred "poll big" sub test, lebench executes > > "context switch" sub test. For the test, it sets the cpu affinity[2] and > > process priority[3] of itself to '0' and '-20', respectively. However, it > > doesn't restore the values to original value even after the "context switch" is > > finished. For the reason, "select big" sub test also run binded on CPU 0 and > > has lowest nice value. Therefore, it can disturb the RCU callback thread for > > the CPU 0, which processes the deferred deallocations of the sockets, and as a > > result it triggers the OOM. > > > > We confirmed the problem disappears by offloading the RCU callbacks from the > > CPU 0 using rcu_nocbs=0 boot parameter or simply restoring the affinity and/or > > priority. > > > > Someone _might_ still argue that this is kernel problem because the problem > > didn't occur on the old kernels prior to the Al's patches. However, setting > > the affinity and priority was available because the program received the > > permission. Therefore, it would be reasonable to blame the system > > administrators rather than the kernel. > > > > So, please ignore this patchset, apology for making confuse. If you still has > > some doubts or need more tests, please let me know. > > > > [1] https://github.com/LinuxPerfStudy/LEBench > > [2] https://github.com/LinuxPerfStudy/LEBench/blob/master/TEST_DIR/OS_Eval.c#L820 > > [3] https://github.com/LinuxPerfStudy/LEBench/blob/master/TEST_DIR/OS_Eval.c#L822 > > Thank you for chasing this down! > > I have had this sort of thing on my list as a potential issue, but given > that it is now really showing up, it sounds like it is time to bump > up its priority a bit. Of course there are limits, so if userspace is > running at any of the real-time priorities, making sufficient CPU time > available to RCU's kthreads becomes userspace's responsibility. But if > everything is running at SCHED_OTHER (which is this case here, correct?), Correct. > then it is reasonable for RCU to do some work to avoid this situation. That would be also great! > > But still, yes, the immediate job is fixing the benchmark. ;-) Totally agreed. > > Thanx, Paul > > PS. Why not just attack all potential issues on my list? Because I > usually learn quite a bit from seeing the problem actually happen. > And sometimes other changes in RCU eliminate the potential issue > before it has a chance to happen. Sounds interesting, I will try some of those in my spare time ;) Thanks, SeongJae Park
