Re: [PATCH tip/core/rcu 1/2] rcu: Add callback-free CPUs

Frederic Weisbecker
2012-10-31T14:10:09Z

2012/10/31 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> +/*
> + * Per-rcu_data kthread, but only for no-CBs CPUs. Each kthread invokes
> + * callbacks queued by the corresponding no-CBs CPU.
> + */
> +static int rcu_nocb_kthread(void *arg)
> +{
> + int c, cl;
> + struct rcu_head *list;
> + struct rcu_head *next;
> + struct rcu_head **tail;
> + struct rcu_data *rdp = arg;
> +
> + /* Each pass through this loop invokes one batch of callbacks */
> + for (;;) {
> + /* If not polling, wait for next batch of callbacks. */
> + if (!rcu_nocb_poll)
> + wait_event(rdp->nocb_wq, rdp->nocb_head);
> + list = ACCESS_ONCE(rdp->nocb_head);
> + if (!list) {
> + schedule_timeout_interruptible(1);
> + continue;
> + }
> +
> + /*
> + * Extract queued callbacks, update counts, and wait
> + * for a grace period to elapse.
> + */
> + ACCESS_ONCE(rdp->nocb_head) = NULL;
> + tail = xchg(&rdp->nocb_tail, &rdp->nocb_head);
> + c = atomic_long_xchg(&rdp->nocb_q_count, 0);
> + cl = atomic_long_xchg(&rdp->nocb_q_count_lazy, 0);
> + ACCESS_ONCE(rdp->nocb_p_count) += c;
> + ACCESS_ONCE(rdp->nocb_p_count_lazy) += cl;
> + wait_rcu_gp(rdp->rsp->call_remote);
> +
> + /* Each pass through the following loop invokes a callback. */
> + trace_rcu_batch_start(rdp->rsp->name, cl, c, -1);
> + c = cl = 0;
> + while (list) {
> + next = list->next;
> + /* Wait for enqueuing to complete, if needed. */
> + while (next == NULL && &list->next != tail) {
> + schedule_timeout_interruptible(1);
> + next = list->next;
> + }
> + debug_rcu_head_unqueue(list);
> + local_bh_disable();
> + if (__rcu_reclaim(rdp->rsp->name, list))
> + cl++;
> + c++;
> + local_bh_enable();
> + list = next;
> + }
> + trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
> + ACCESS_ONCE(rdp->nocb_p_count) -= c;
> + ACCESS_ONCE(rdp->nocb_p_count_lazy) -= cl;
> + rdp->n_cbs_invoked += c;
> + }
> + return 0;
> +}
> +
> +/* Initialize per-rcu_data variables for no-CBs CPUs. */
> +static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
> +{
> + rdp->nocb_tail = &rdp->nocb_head;
> + init_waitqueue_head(&rdp->nocb_wq);
> +}
> +
> +/* Create a kthread for each RCU flavor for each no-CBs CPU. */
> +static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
> +{
> + int cpu;
> + struct rcu_data *rdp;
> + struct task_struct *t;
> +
> + if (rcu_nocb_mask == NULL)
> + return;
> + for_each_cpu(cpu, rcu_nocb_mask) {
> + rdp = per_cpu_ptr(rsp->rda, cpu);
> + t = kthread_run(rcu_nocb_kthread, rdp, "rcuo%d", cpu);

Sorry, I think I left my brain in the middle of the diff. But there is
something I'm misunderstanding I think. Here you're creating an
rcu_nocb_kthread per nocb cpu. Looking at the code of
rcu_nocb_kthread(), it seems to execute the callbacks with
__rcu_reclaim().

So, in the end, no callbacks CPU execute their callbacks. Isn't it the
opposite than what is expected? (again, just referring to my
misunderstanding).

Thanks.

> + BUG_ON(IS_ERR(t));
> + ACCESS_ONCE(rdp->nocb_kthread) = t;
> + }
> +}