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










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