On 29/04/24 07:54, Sven Schnelle wrote: > The current implementation of available_idle_cpu() doesn't test > whether a possible cpu is offline. On s390 this dereferences a > NULL pointer in arch_vcpu_is_preempted() because lowcore is not > allocated for offline cpus. On x86, tracing also shows calls to > available_idle_cpu() after a cpu is disabled, but it looks like > this isn't causing any (obvious) issue. Nevertheless, add a check > and return early if the cpu isn't online. > > Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx> So most of the uses of that function is in wakeup task placement. o find_idlest_cpu() works on the sched_domain spans, so shouldn't deal with offline CPUs. o select_idle_sibling() may issue an available_idle_cpu(prev) with an offline previous, which would trigger your issue. Currently, even if select_idle_sibling() picks an offline CPU, this will get corrected by select_fallback_rq() at the end of select_task_rq(). However, it would make sense to realize @prev isn't a suitable pick before making it to the fallback machinery, in which case your patch makes sense beyond just fixing s390. Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>
