Re: [PATCH 4/4] workqueue: handle change in cpu-node relationship.

From: Kamezawa Hiroyuki
Date: Sun Dec 14 2014 - 23:23:04 EST

(2014/12/15 12:34), Lai Jiangshan wrote:
On 12/15/2014 10:55 AM, Kamezawa Hiroyuki wrote:
(2014/12/15 11:48), Lai Jiangshan wrote:
On 12/15/2014 10:20 AM, Kamezawa Hiroyuki wrote:
(2014/12/15 11:12), Lai Jiangshan wrote:
On 12/14/2014 12:38 AM, Kamezawa Hiroyuki wrote:
Although workqueue detects relationship between cpu<->node at boot,
it is finally determined in cpu_up().
This patch tries to update pool->node using online status of cpus.

1. When a node goes down, clear per-cpu pool's node attr.
2. When a cpu comes up, update per-cpu pool's node attr.
3. When a cpu comes up, update possinle node cpumask workqueue is using for sched.
4. Detect the best node for unbound pool's cpumask using the latest info.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
kernel/workqueue.c | 67 ++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 53 insertions(+), 14 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 07b4eb5..259b3ba 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -266,7 +266,8 @@ struct workqueue_struct {
static struct kmem_cache *pwq_cache;

static cpumask_var_t *wq_numa_possible_cpumask;
- /* possible CPUs of each node */
+ /* possible CPUs of each node initialized with possible info at boot.
+ but modified at cpu hotplug to be adjusted to real info. */

static bool wq_disable_numa;
module_param_named(disable_numa, wq_disable_numa, bool, 0444);
@@ -3449,6 +3450,31 @@ static void put_unbound_pool(struct worker_pool *pool)
call_rcu_sched(&pool->rcu, rcu_free_pool);
}

+/*
+ * detect best node for given cpumask.
+ */
+static int pool_detect_best_node(const struct cpumask *cpumask)
+{
+ int node, best, match, selected;
+ static struct cpumask andmask; /* we're under mutex */
+
+ /* Is any node okay ? */
+ if (!wq_numa_enabled ||
+ cpumask_subset(cpu_online_mask, cpumask))
+ return NUMA_NO_NODE;
+ best = 0;
+ selected = NUMA_NO_NODE;
+ /* select a node which contains the most cpu of cpumask */
+ for_each_node_state(node, N_ONLINE) {
+ cpumask_and(&andmask, cpumask, cpumask_of_node(node));
+ match = cpumask_weight(&andmask);
+ if (match > best)
+ selected = node;
+ }
+ return selected;
+}
+
+
/**
* get_unbound_pool - get a worker_pool with the specified attributes
* @attrs: the attributes of the worker_pool to get
@@ -3467,7 +3493,6 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
{
u32 hash = wqattrs_hash(attrs);
struct worker_pool *pool;
- int node;

lockdep_assert_held(&wq_pool_mutex);

@@ -3492,17 +3517,7 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
* 'struct workqueue_attrs' comments for detail.
*/
pool->attrs->no_numa = false;
-
- /* if cpumask is contained inside a NUMA node, we belong to that node */
- if (wq_numa_enabled) {
- for_each_node(node) {
- if (cpumask_subset(pool->attrs->cpumask,
- wq_numa_possible_cpumask[node])) {
- pool->node = node;
- break;
- }
- }
- }
+ pool->node = pool_detect_best_node(pool->attrs->cpumask);

if (worker_pool_assign_id(pool) < 0)
goto fail;
@@ -4567,7 +4582,7 @@ static int workqueue_cpu_up_callback(struct notifier_block *nfb,
int cpu = (unsigned long)hcpu;
struct worker_pool *pool;
struct workqueue_struct *wq;
- int pi;
+ int pi, node;

switch (action & ~CPU_TASKS_FROZEN) {
case CPU_UP_PREPARE:
@@ -4583,6 +4598,16 @@ static int workqueue_cpu_up_callback(struct notifier_block *nfb,
case CPU_ONLINE:
mutex_lock(&wq_pool_mutex);

+ /* now cpu <-> node info is established, update the info. */
+ if (!wq_disable_numa) {



+ for_each_node_state(node, N_POSSIBLE)
+ cpumask_clear_cpu(cpu,
+ wq_numa_possible_cpumask[node]);

The wq code try to reuse the origin pwqs/pools when the node still have cpu online.
these 3 lines of code will cause the origin pwqs/pools be on the road of dying, and
create a new set of pwqs/pools.

because the result of wq_calc_node_cpumask() changes ?

Yes.

Do you mean some comment should be added here ? or explaination for your reply for [3/4] ?

this fix [4/4] breaks the original design.


I'm sorry that I can't understand what this patch breaks.

the pwqs/pools should be kept if the node still have cpu online.

So, the fix's grand design should be

1. drop old pwq/pools only at node offline.
2. set proper pool->node based on online node info.
3. update pool->node of per-cpu-pool at cpu ONLINE.

Hm. (1) is done because cpumask_of_node() turns to be zero-filled
after all cpus on a node offlined.

But, cpu-to-node relationship cannot be available until a cpu get onlined.
It changes at every cpu onlining. So, at node online, refleshing cpumasks
of workqueues only after _all_ cpus on node are onlined seems to be the only way
but I'm not sure how to get a mask of possible cpus on a node.

Possible another way may be using auto numa balancing's numa node hint for worker scheduling.

Do you have any idea ?

Thanks,
-Kame










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