Valentin Schneider <vschneid@xxxxxxxxxx> writes: > 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> Thanks for the review! Ingo/Peter, gentle ping, are you planning to take this patch?
