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

[Lai Jiangshan]

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@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

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.

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]);
+	}


> 
> Possible another way may be using auto numa balancing's numa node hint for worker scheduling.
> 
> Do you have any idea ?

Is it hard to keep the cpu-node relationship unchanged?
You know that, all cpu ids, node ids are soft-ware logical id, we can change relationship
between logic id and physical id but keep relationship among logical ids unchanged.

> 
> Thanks,
> -Kame
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> .
> 

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