Hey Joel On 09/23/23 01:14, Joel Fernandes (Google) wrote: > During RCU-boost testing with the TREE03 rcutorture config, I found that > after a few hours, the machine locks up. > > On tracing, I found that there is a live lock happening between 2 CPUs. > One CPU has an RT task running, while another CPU is being offlined > which also has an RT task running. During this offlining, all threads > are migrated. The migration thread is repeatedly scheduled to migrate > actively running tasks on the CPU being offlined. This results in a live > lock because select_fallback_rq() keeps picking the CPU that an RT task > is already running on only to get pushed back to the CPU being offlined. > > It is anyway pointless to pick CPUs for pushing tasks to if they are > being offlined only to get migrated away to somewhere else. This could > also add unwanted latency to this task. > > Fix these issues by not selecting CPUs in RT if they are not 'active' > for scheduling, using the cpu_active_mask. Other parts in core.c already > use cpu_active_mask to prevent tasks from being put on CPUs going > offline. I think this is the same report as this one from Xuewen https://lore.kernel.org/lkml/20221114120453.3233-1-xuewen.yan@xxxxxxxxxx/ You also have only 2 CPUs system, so when one CPU is offline, the other becomes overloaded. I think it is worth trying to fix rto_next_cpu() to consider the cpu_active_mask too (see my reply to Xuewen then). Keep your change as well. I think Xuewen and I considered disabling the overloaded logic when we devolve to a UP system too as the whole logic doesn't make sense anymore then. This is more of an optimization than correctness though. Don't feel strongly about it now though. Thanks! -- Qais Yousef > > Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > kernel/sched/cpupri.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c > index a286e726eb4b..42c40cfdf836 100644 > --- a/kernel/sched/cpupri.c > +++ b/kernel/sched/cpupri.c > @@ -101,6 +101,7 @@ static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p, > > if (lowest_mask) { > cpumask_and(lowest_mask, &p->cpus_mask, vec->mask); > + cpumask_and(lowest_mask, lowest_mask, cpu_active_mask); > > /* > * We have to ensure that we have at least one bit > -- > 2.42.0.515.g380fc7ccd1-goog >
