On Tue, Apr 20, 2021 at 04:20:56PM +0200, Peter Zijlstra wrote: > On Tue, Apr 20, 2021 at 10:46:33AM +0100, Vincent Donnefort wrote: > > > Found the issue: > > > > $ cat hotplug/states: > > 219: sched:active > > 220: online > > > > CPU0: > > > > $ echo 219 > hotplug/fail > > $ echo 0 > online > > > > => cpu_active = 1 cpu_dying = 1 > > > > which means that later on, for another CPU hotunplug, in > > __balance_push_cpu_stop(), the fallback rq for a kthread can select that > > CPU0, but __migrate_task() would fail and we end-up in an infinite loop, > > trying to migrate that task to CPU0. > > > > The problem is that for a failure in sched:active, as "online" has no > > callback, > > there will be no call to cpuhp_invoke_callback(). Hence, the cpu_dying bit > > would > > not be reset. > > Urgh! Good find. > > > Maybe cpuhp_reset_state() and cpuhp_set_state() would then be a better > > place to > > switch the dying bit? > > Yes, except now cpuhp_invoke_ap_callback() makes my head hurt, that runs > the callbacks out of order. I _think_ we can ignore it, but .... > > Something like the below, let me see if I can reproduce and test.
I seem to have triggered the BUG() in select_fallback_rq() with your recipie. Have cpu0 fail on sched:active, then offline all other CPUs. Now lemme add that patch.
