On Mon, Sep 11, 2023 at 01:17:30PM +0000, Joel Fernandes wrote: > On Mon, Sep 11, 2023 at 01:16:21AM -0700, Paul E. McKenney wrote: > > On Mon, Sep 11, 2023 at 02:27:25AM +0000, Joel Fernandes wrote: > > > On Sun, Sep 10, 2023 at 07:37:13PM -0400, Joel Fernandes wrote: > > > > On Sun, Sep 10, 2023 at 5:16 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > > > > > > > On Sun, Sep 10, 2023 at 08:14:45PM +0000, Joel Fernandes wrote: > > > > [...] > > > > > > > I have been running into another intermittent one as well which > > > > > > > is the boost failure and that happens once in 10-15 runs or so. > > > > > > > > > > > > > > I was thinking of running the following configuration on an automated > > > > > > > regular basis to at least provide a better clue on the lucky run that > > > > > > > catches an issue. But then the issue is it would change timing enough > > > > > > > to maybe hide bugs. I could also make it submit logs automatically to > > > > > > > the list on such occurrences, but one step at a time and all that. I > > > > > > > do need to add (hopefully less noisy) tick/timer related trace events. > > > > > > > > > > > > > > # Define the bootargs array > > > > > > > bootargs=( > > > > > > > "ftrace_dump_on_oops" > > > > > > > "panic_on_warn=1" > > > > > > > "sysctl.kernel.panic_on_rcu_stall=1" > > > > > > > "sysctl.kernel.max_rcu_stall_to_panic=1" > > > > > > > "trace_buf_size=10K" > > > > > > > "traceoff_on_warning=1" > > > > > > > "panic_print=0x1f" # To dump held locks, mem and other info. > > > > > > > ) > > > > > > > # Define the trace events array passed to bootargs. > > > > > > > trace_events=( > > > > > > > "sched:sched_switch" > > > > > > > "sched:sched_waking" > > > > > > > "rcu:rcu_callback" > > > > > > > "rcu:rcu_fqs" > > > > > > > "rcu:rcu_quiescent_state_report" > > > > > > > "rcu:rcu_grace_period" > > > > > > > ) > > > > > > > > > > > > So some insight on this boost failure. Just before the boost failures are > > > > > > reported, I see the migration thread interferring with the rcu_preempt thread > > > > > > (aka GP kthread). See trace below. Of note is that the rcu_preempt thread is > > > > > > runnable while context switching, which means its execution is interferred. > > > > > > The rcu_preempt thread is at RT prio 2 as can be seen. > > > > > > > > > > > > So some open-ended questions: what exactly does the migration thread want, > > > > > > this is something related to CPU hotplug? And if the migration thread had to > > > > > > run, why did the rcu_preempt thread not get pushed to another CPU by the > > > > > > scheduler? We have 16 vCPUs for this test. > > > > > > > > > > Maybe we need a cpus_read_lock() before doing a given boost-test interval > > > > > and a cpus_read_unlock() after finishing one? But much depends on > > > > > exactly what is starting those migration threads. > > > > > > > > But in the field, a real RT task can preempt a reader without doing > > > > cpus_read_lock() and may run into a similar boost issue? > > > > The sysctl_sched_rt_runtime should prevent a livelock in most > > configurations. Here, rcutorture explicitly disables this. > > I see. Though RT throttling will actually stall the rcu_preempt thread as > well in the real world. RT throttling is a bit broken and we're trying to fix > it in scheduler land. Even if there are idle CPUs, RT throttling will starve > not just the offending RT task, but all of them essentially causing a > priority inversion between running RT and CFS tasks. Fair point. But that requires that the offending runaway RT task hit both a reader and the grace-period kthread. Keeping in mind that rcutorture is provisioning one runaway RT task per CPU, which in the real world is hopefully quite rare. Hopefully. ;-) > > > > > Then again, TREE03 is pretty aggressive about doing CPU hotplug. > > > > > > > > Ok. I put a trace_printk() in the stopper thread to see what the > > > > ->fn() is. I'm doing another run to see what falls out. > > > > > > I think I am getting somewhere, It looks like the migration thread tries to > > > push the task out due to __balance_push_cpu_stop. Clearly it fails because > > > the rcu_preempt task is still on CPU 3. And the whole thing is live locked > > > retrying the same. > > > > > > I am not sure if the migration thread is trying to migrate rcu_preempt > > > because the CPU is going down or because a higher priority RT task is > > > running. > > > > > > rcu_pree-13 3d..4. 2936723336us : sched_waking: comm=migration/3 pid=34 prio=0 target_cpu=003 > > > rcu_pree-13 3d..2. 2936723338us : sched_switch: prev_comm=rcu_preempt prev_pid=13 prev_prio=97 prev_state=R ==> next_comm=migration/3 next_pid=34 next_prio=0 > > > migratio-34 3..... 2936723341us : cpu_stopper_thread: cpu_stop: caller=balance_push+0xc9/0x150 fn=__balance_push_cpu_stop+0x0/0x1c0 on cpu 3 > > > migratio-34 3d..2. 2936723345us : sched_switch: prev_comm=migration/3 prev_pid=34 prev_prio=0 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120 > > > <idle>-0 3d..2. 2936723676us : sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=13 next_prio=97 > > > rcu_pree-13 3d..4. 2936723678us : sched_waking: comm=migration/3 pid=34 prio=0 target_cpu=003 > > > rcu_pree-13 3d..2. 2936723680us : sched_switch: prev_comm=rcu_preempt prev_pid=13 prev_prio=97 prev_state=R ==> next_comm=migration/3 next_pid=34 next_prio=0 > > > migratio-34 3..... 2936723683us : cpu_stopper_thread: cpu_stop: caller=balance_push+0xc9/0x150 fn=__balance_push_cpu_stop+0x0/0x1c0 on cpu 3 > > > > > > I am digging deeper to see why the rcu_preempt thread cannot be pushed out > > > and then I'll also look at why is it being pushed out in the first place. > > > > > > At least I have a strong repro now running 5 instances of TREE03 in parallel > > > for several hours. > > > > Very good! Then why not boot with rcutorture.onoff_interval=0 and see if > > the problem still occurs? If yes, then there is definitely some reason > > other than CPU hotplug that makes this happen. > > Sure, I'll do this now. If that disappears, I'll do the cpu_read_lock() > suggestion you mentioned. IMHO this is increasingly looking like a scheduling > issue with RT and too many RT tasks running at the same time where the > rcu_preempt thread migrates around looking for a CPU than it does any work. > > Today morning's traces show that __balance_push_cpu_stop() actually > successfully moves the rcu_preempt task out of the CPU. But a few ms later, > it is back on the same CPU again only to have the migration thread try to > move it out again. Maybe the cpuhp threads are fighting with it. There's also > all the rcutorture boost kthreads that are all running at the same time > during the failure but those are lower priority than the rcu_preempt thread > in this test. > > Also I am switching to some day job things while the onoff_interval=0 test > runs, thanks a lot Paul for your help! I'll continue to look at this as I > can. Sounds like good progress! Please let me know how it goes!!! Thanx, Paul
