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

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2014/12/15 14:19), Lai Jiangshan wrote:
> On 12/15/2014 12:04 PM, Kamezawa Hiroyuki wrote:
>> (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@jp.fujitsu.com>
>>>>>>>> ---
>>>>>>>>      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
>
> It changes at every cpu being added which earlier than any cpu of the node will be online.
>
>> of workqueues only after _all_ cpus on node are onlined seems to be the only way
>
> When _any_ cpu on the new node is onlining, _add_ cpus of the node were *added*, which means
> all cpu_to_node()s of the all cpus on new node ware updated.
>

Thank you, I misuderstood that.


> so we can check the updated information when up online.  This is what my patchset did.
>
>> but I'm not sure how to get a mask of possible cpus on a node.
>
> like this:
>
> +	for_each_possible_cpu(cpu) {
> +		node = cpu_to_node(node);
> +		if (node == new_node)
> +			cpumask_set_cpu(cpu, wq_numa_possible_cpumask[new_node]);
> +	}
>

Okay, I talked with Ishimatu-san and get some more knowledege.

let me clarify. not at cpu hotplug,

(A) At physical cpu hot add, cpu <-> node relationship is established.
(B) If an exisitng cpu was on a memory less node, cpu<->node relationship may change
     at CPU_ONLINE.

Because (2) will not change topology of cpuids, this doesn't affect cpumask for sched.
So, we just need to handle following.

1. update pc->node if necessary.
2. update wq_numa_possible_mask[] at physical cpu hot-add.

Leave wq_numa_possible_mask as it is for case (B) because the change doesn't affect
the affinity group of cpus. no affects to cpumask.

I'll cook a patch and do a test.

Thanks,
-Kame