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

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Date:	Fri, 2 Nov 2012 11:41:32 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, sbw@....edu,
	patches@...aro.org, joe.korty@...r.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Add callback-free CPUs

On Wed, Oct 31, 2012 at 03:10:04PM +0100, Frederic Weisbecker wrote:
> 2012/10/31 Paul E. McKenney <paulmck@...ux.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().

True, executing within the context of the kthread, which might be
executing on any CPU.

> 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).

The no-callbacks CPU would execute its own callbacks only if the
corresponding kthread happened to be executing on that CPU.
If you wanted a given CPU to be completely free of callbacks,
you would instead constrain the corresponding kthread to run
elsewhere.

							Thanx, Paul

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

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