On Thu, Jan 21, 2021 at 02:01:03PM +0000, Valentin Schneider wrote: > On 21/01/21 11:17, Peter Zijlstra wrote: > > @@ -7504,6 +7525,9 @@ int sched_cpu_deactivate(unsigned int cp > > * preempt-disabled and RCU users of this state to go away such that > > * all new such users will observe it. > > * > > + * Specifically, we rely on ttwu to no longer target this CPU, see > > + * ttwu_queue_cond() and is_cpu_allowed(). > > + * > > So the last time ttwu_queue_wakelist() can append a task onto a dying > CPU's wakelist is before sched_cpu_deactivate()'s synchronize_rcu() > returns. > > As discussed on IRC, paranoia would have us issue a > > flush_smp_call_function_from_idle() > > upon returning from said sync, but this will require further surgery.
Right, specifically RCU needs a little more help there. > Do we want something like the below in the meantime? Ideally we'd warn on > setting rq->ttwu_pending when !cpu_active(), but as per the above this is > allowed before the synchronize_rcu() returns. I'm not sure I'm brave enough to add that just now :/
