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SubjectRe: [PATCH 4/4] numa: introduce numa cling feature
On Wed, Jul 03, 2019 at 11:34:16AM +0800, 王贇 wrote:
> Although we paid so many effort to settle down task on a particular
> node, there are still chances for a task to leave it's preferred
> node, that is by wakeup, numa swap migrations or load balance.
>
> When we are using cpu cgroup in share way, since all the workloads
> see all the cpus, it could be really bad especially when there
> are too many fast wakeup, although now we can numa group the tasks,
> they won't really stay on the same node, for example we have numa
> group ng_A, ng_B, ng_C, ng_D, it's very likely result as:
>
> CPU Usage:
> Node 0 Node 1
> ng_A(600%) ng_A(400%)
> ng_B(400%) ng_B(600%)
> ng_C(400%) ng_C(600%)
> ng_D(600%) ng_D(400%)
>
> Memory Ratio:
> Node 0 Node 1
> ng_A(60%) ng_A(40%)
> ng_B(40%) ng_B(60%)
> ng_C(40%) ng_C(60%)
> ng_D(60%) ng_D(40%)
>
> Locality won't be too bad but far from the best situation, we want
> a numa group to settle down thoroughly on a particular node, with
> every thing balanced.
>
> 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.

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