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    SubjectRe: [PATCH] workqueue: don't alloc_percpu for single workqueue
    On 01/21, Lai Jiangshan wrote:
    >
    > allocating memory for every cpu for single workqueue is waste.

    Yes, perhaps this makes sense, we can save a bit of per-cpu memory
    for each single-threaded wq, and the patch looks correct.

    > -static struct cpu_workqueue_struct *
    > -init_cpu_workqueue(struct workqueue_struct *wq, int cpu)
    > +static void init_cpu_workqueue(struct workqueue_struct *wq,
    > + struct cpu_workqueue_struct *cwq)
    > {
    > - struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
    > -
    > cwq->wq = wq;
    > spin_lock_init(&cwq->lock);
    > INIT_LIST_HEAD(&cwq->worklist);
    > init_waitqueue_head(&cwq->more_work);
    > -
    > - return cwq;
    > }

    Do we really need to change the prototype of init_cpu_workqueue()
    and change then change __create_workqueue_key() accordingly?
    Afaics, the only change in init_cpu_workqueue() we need is

    - struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
    + struct cpu_workqueue_struct *cwq = wq_per_cpu(wq, cpu);

    no?

    > @@ -906,6 +907,13 @@ void destroy_workqueue(struct workqueue_struct *wq)
    > const struct cpumask *cpu_map = wq_cpu_map(wq);
    > int cpu;
    >
    > + if (is_wq_single_threaded(wq)) {
    > + cleanup_workqueue_thread(wq->cpu_wq);
    > + kfree(wq->cpu_wq);
    > + kfree(wq);
    > + return;
    > + }

    again, not sure I understand why this change is needed. Afaics we
    only need to use kfree(wq->cpu_wq) instead of free_percpu() if
    it is single-threaded.

    Oleg.



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