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    SubjectRe: [PATCH 2/6] workqueue: add support for lazy workqueues
    On Thu, Aug 20 2009, Andrew Morton wrote:
    > On Thu, 20 Aug 2009 12:17:39 +0200
    > Jens Axboe <jens.axboe@oracle.com> wrote:
    >
    > > Lazy workqueues are like normal workqueues, except they don't
    > > start a thread per CPU by default. Instead threads are started
    > > when they are needed, and exit when they have been idle for
    > > some time.
    > >
    > >
    > > ...
    > >
    > > @@ -280,7 +309,34 @@ static void run_workqueue(struct cpu_workqueue_struct *cwq)
    > > trace_workqueue_execution(cwq->thread, work);
    > > cwq->current_work = work;
    > > list_del_init(cwq->worklist.next);
    > > + cpu = smp_processor_id();
    > > spin_unlock_irq(&cwq->lock);
    > > + did_work = 1;
    > > +
    > > + /*
    > > + * If work->cpu isn't us, then we need to create the target
    > > + * workqueue thread (if someone didn't already do that) and
    > > + * move the work over there.
    > > + */
    > > + if ((cwq->wq->flags & WQ_F_LAZY) && work->cpu != cpu) {
    > > + struct cpu_workqueue_struct *__cwq;
    > > + struct task_struct *p;
    > > + int err;
    > > +
    > > + __cwq = wq_per_cpu(cwq->wq, work->cpu);
    > > + p = __cwq->thread;
    > > + if (!p)
    > > + err = create_workqueue_thread(__cwq, work->cpu);
    > > + p = __cwq->thread;
    > > + if (p) {
    > > + if (work->cpu >= 0)
    >
    > It's an unsigned int. This test is always true.
    >
    > > + kthread_bind(p, work->cpu);
    >
    > I wonder what happens if work->cpu isn't online any more.

    That's a good question. The workqueue "documentation" states that it is
    the callers responsibility to ensure that the CPU stays online, but I
    think that requirement is pretty much ignored. Probably since it'd be
    costly to do.

    So that bits needs looking into.

    --
    Jens Axboe



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