On Wed, 6 May 2020 07:41:51 -0700 "Paul E. McKenney" paulmck@kernel.org 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#L82... [3] https://github.com/LinuxPerfStudy/LEBench/blob/master/TEST_DIR/OS_Eval.c#L82...
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, PaulPS. 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