On Fri, Jul 12, 2019 at 11:10:08AM +0800, 王贇 wrote: > On 2019/7/11 下午10:27, Peter Zijlstra wrote: > >> Thus we introduce the numa cling, which try to prevent tasks leaving > >> the preferred node on wakeup fast path. > > > > > >> @@ -6195,6 +6447,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > >> if ((unsigned)i < nr_cpumask_bits) > >> return i; > >> > >> + /* > >> + * Failed to find an idle cpu, wake affine may want to pull but > >> + * try stay on prev-cpu when the task cling to it. > >> + */ > >> + if (task_numa_cling(p, cpu_to_node(prev), cpu_to_node(target))) > >> + return prev; > >> + > >> return target; > >> } > > > > Select idle sibling should never cross node boundaries and is thus the > > entirely wrong place to fix anything. > > Hmm.. in our early testing the printk show both select_task_rq_fair() and > task_numa_find_cpu() will call select_idle_sibling with prev and target on > different node, thus we pick this point to save few lines. But it will never return @prev if it is not in the same cache domain as @target. See how everything is gated by: && cpus_share_cache(x, target) > But if the semantics of select_idle_sibling() is to return cpu on the same > node of target, what about move the logical after select_idle_sibling() for > the two callers? No, that's insane. You don't do select_idle_sibling() to then ignore the result. You have to change @target before calling select_idle_sibling().
