Hi Paul, Thanks for your inputs. On Wed, 16 Dec 2020 at 21:33, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Wed, Dec 16, 2020 at 03:40:04PM +0530, Naresh Kamboju wrote: > > Linux Kernel Functional Testing (LKFT) started running rcu-torture tests on > > qemu_arm64, qemu_arm qemu_x86_64 and qemu_i386 from our CI build systems. > > > > The following warning(s) noticed on qemu_i386 while running rcu-torture test > > on Linux mainline and Linux -next master branch. Since we do not have baseline > > results i can not comment this as regression but when compared with > > stable-rc 5.4 kernel this warning is new on mainline and next. > > The rcutorture testing "stutters", that is, it periodically intentionally > drops the test load down to zero for a few seconds. The expectation is > that with no load, rcutorture will have no trouble finishing any needed > grace periods within that zero-load period. If at the end of the stutter > period, RCU work remains undone, then this warning is emitted. > > This warning can be a false positive in the following situations: > > 1. The system on which you are running rcutorture is under > additional heavy load. The DUT is running the test - rcutorture - only. > 2. You are running multiple guest OSes, each of which is running > rcutorture, and vCPUs from each of the guest OSes ends up > sharing a core with a vCPU from one of the other guests. This > can cause the zero-load period to not be so unloaded. > > 3. You built rcutorture into your kernel, so that rcutorture starts > immediately at boot time (CONFIG_RCU_TORTURE_TEST=y). If your > boot takes long enough, rcutorture can massively overload the > single boot CPU, which can in turn result in this warning. The test was built as a module. CONFIG_RCU_TORTURE_TEST=m > > If you are in situation #1, I suggest disabling stuttering using the > rcutorture.stutter=0 kernel boot parameter. > > If you are in situation #2, I suggest binding the guest-OS vCPUs > to avoid them sharing cores with each other. > > If you are in situation #3, I have patches that I expect to submit > upstream in the v5.12 merge window that can help. Hey, they work for me! > If you would like to test them before then, please let me know. > > If something else is going on, please let me know what it is so that > I can fix it one way or another. We were running on qemu_i386 today. I have tested on real hardware and the reported problem has been reproduced. > This warning has been present for quite some time, but I continually > make rcutorture more aggressive, and this could well be part of the > fallout of additional rcutorture aggression. > > And either way, thank you for trying out rcutorture! We are happy to test :) - Naresh
