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SubjectRe: [PATCH 4/4] numa: introduce numa cling feature
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().

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