Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

Paul E. McKenney
2014-08-07T06:45:00Z

On Tue, Aug 05, 2014 at 05:51:01PM -0700, Paul E. McKenney wrote:
> On Wed, Aug 06, 2014 at 08:33:29AM +0800, Lai Jiangshan wrote:
> > On 08/06/2014 05:55 AM, Paul E. McKenney wrote:
> > > On Tue, Aug 05, 2014 at 08:47:55AM +0800, Lai Jiangshan wrote:
> > >> On 08/04/2014 10:56 PM, Peter Zijlstra wrote:
> > >>> On Mon, Aug 04, 2014 at 02:25:15PM +0200, Peter Zijlstra wrote:
> > >>>> On Mon, Aug 04, 2014 at 04:50:44AM -0700, Paul E. McKenney wrote:
> > >>>>> OK, I will bite...
> > >>>>>
> > >>>>> What kinds of tasks are on a runqueue, but neither ->on_cpu nor
> > >>>>> PREEMPT_ACTIVE?
> > >>>>
> > >>>> Userspace tasks, they don't necessarily get PREEMPT_ACTIVE when
> > >>>> preempted. Now obviously you're not _that_ interested in userspace tasks
> > >>>> for this, so that might be ok.
> > >>>>
> > >>>> But the main point was, you cannot use ->on_cpu or PREEMPT_ACTIVE
> > >>>> without holding rq->lock.
> > >>>
> > >>> Hmm, maybe you can, we have the context switch in between setting
> > >>> ->on_cpu and clearing PREEMPT_ACTIVE and vice-versa.
> > >>>
> > >>> The context switch (obviously) provides a full barrier, so we might be
> > >>> able to -- with careful consideration -- read these two separate values
> > >>> and construct something usable from them.
> > >>>
> > >>> Something like:
> > >>>
> > >>> task_preempt_count(tsk) & PREEMPT_ACTIVE
> > >> the @tsk is running on_cpu, the above result is false.
> > >>> smp_rmb();
> > >>> tsk->on_cpu
> > >> now the @tsk is preempted, the above result also is false.
> > >>
> > >> so it is useless if we fetch the preempt_count and on_cpu in two separated
> > >> instructions. Maybe it would work if we also take tsk->nivcsw in consideration.
> > >> (I just noticed that tsk->n[i]vcsw are the version numbers for the tsk->on_cpu)
> > >>
> > >> bool task_on_cpu_or_preempted(tsk)
> > >> {
> > >> unsigned long saved_nivcsw;
> > >>
> > >> saved_nivcsw = task->nivcsw;
> > >> if (tsk->on_cpu)
> > >> return true;
> > >>
> > >> smp_rmb();
> > >>
> > >> if (task_preempt_count(tsk) & PREEMPT_ACTIVE)
> > >> return true;
> > >>
> > >> smp_rmb();
> > >>
> > >> if (tsk->on_cpu || task->nivcsw != saved_nivcsw)
> > >> return true;
> > >>
> > >> return false;
> > >> }
> > >>
> > >>>
> > >>> And because we set PREEMPT_ACTIVE before clearing on_cpu, this should
> > >>> race the right way (err towards the inclusive side).
> > >>>
> > >>> Obviously that wants a big fat comment...
> > >
> > > How about the following? Non-nohz_full userspace tasks are already covered
> > > courtesy of scheduling-clock interrupts, and this handles nohz_full usermode
> > > tasks.
> > >
> > > Thoughts?
> > >
> > > Thanx, Paul
> > >
> > > ------------------------------------------------------------------------
> > >
> > > rcu: Make TASKS_RCU handle nohx_full= CPUs
> > >
> > > Currently TASKS_RCU would ignore a CPU running a task in nohz_full=
> > > usermode execution. There would be neither a context switch nor a
> > > scheduling-clock interrupt to tell TASKS_RCU that the task in question
> > > had passed through a quiescent state. The grace period would therefore
> > > extend indefinitely. This commit therefore makes RCU's dyntick-idle
> > > subsystem record the task_struct structure of the task that is running
> > > in dyntick-idle mode on each CPU. The TASKS_RCU grace period can
> > > then access this information and record a quiescent state on
> > > behalf of any CPU running in dyntick-idle usermode.
> > >
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > >
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index f504f797c9c8..777aac3a34c0 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -1140,5 +1140,14 @@ static inline void rcu_sysidle_force_exit(void)
> > >
> > > #endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > >
> > > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
> > > +struct task_struct *rcu_dynticks_task_cur(int cpu);
> > > +#else /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > > +static inline struct task_struct *rcu_dynticks_task_cur(int cpu)
> > > +{
> > > + return NULL;
> > > +}
> > > +#endif /* #else #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > > +
> > >
> > > #endif /* __LINUX_RCUPDATE_H */
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 645a33efc0d4..86a0a7d5bbbd 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -526,6 +526,7 @@ static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
> > > atomic_inc(&rdtp->dynticks);
> > > smp_mb__after_atomic(); /* Force ordering with next sojourn. */
> > > WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
> > > + rcu_dynticks_task_enter(rdtp, current);
> > >
> > > /*
> > > * It is illegal to enter an extended quiescent state while
> > > @@ -642,6 +643,7 @@ void rcu_irq_exit(void)
> > > static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
> > > int user)
> > > {
> > > + rcu_dynticks_task_exit(rdtp);
> >
> > What happen when the trampoline happens before rcu_eqs_exit_common()?
> > synchronize_sched() also skip this CPUs. I think, for all CPUs, only
> > real schedule is reliable.
>
> True, this prohibits tracing the point from the call to
> rcu_eqs_enter_common() to the transition to usermode. I am betting that
> this is OK, though.

And if not, it is not all that hard to handle this case.

Thanx, Paul

> > > smp_mb__before_atomic(); /* Force ordering w/previous sojourn. */
> > > atomic_inc(&rdtp->dynticks);
> > > /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
> > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > > index 0f69a79c5b7d..1e79fa1b7cbf 100644
> > > --- a/kernel/rcu/tree.h
> > > +++ b/kernel/rcu/tree.h
> > > @@ -88,6 +88,9 @@ struct rcu_dynticks {
> > > /* Process level is worth LLONG_MAX/2. */
> > > int dynticks_nmi_nesting; /* Track NMI nesting level. */
> > > atomic_t dynticks; /* Even value for idle, else odd. */
> > > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
> > > + struct task_struct *dynticks_tsk;
> > > +#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > > #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > > long long dynticks_idle_nesting;
> > > /* irq/process nesting level from idle. */
> > > @@ -579,6 +582,9 @@ static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
> > > static void rcu_bind_gp_kthread(void);
> > > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
> > > static bool rcu_nohz_full_cpu(struct rcu_state *rsp);
> > > +static void rcu_dynticks_task_enter(struct rcu_dynticks *rdtp,
> > > + struct task_struct *t);
> > > +static void rcu_dynticks_task_exit(struct rcu_dynticks *rdtp);
> > >
> > > #endif /* #ifndef RCU_TREE_NONCORE */
> > >
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index a86a363ea453..442d62edc564 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -2852,3 +2852,29 @@ static void rcu_bind_gp_kthread(void)
> > > set_cpus_allowed_ptr(current, cpumask_of(cpu));
> > > #endif /* #ifdef CONFIG_NO_HZ_FULL */
> > > }
> > > +
> > > +/* Record the current task on dyntick-idle entry. */
> > > +static void rcu_dynticks_task_enter(struct rcu_dynticks *rdtp,
> > > + struct task_struct *t)
> > > +{
> > > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
> > > + ACCESS_ONCE(rdtp->dynticks_tsk) = t;
> > > +#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > > +}
> > > +
> > > +/* Record no current task on dyntick-idle exit. */
> > > +static void rcu_dynticks_task_exit(struct rcu_dynticks *rdtp)
> > > +{
> > > + rcu_dynticks_task_enter(rdtp, NULL);
> > > +}
> > > +
> > > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
> > > +struct task_struct *rcu_dynticks_task_cur(int cpu)
> > > +{
> > > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> > > + struct task_struct *t = ACCESS_ONCE(rdtp->dynticks_tsk);
> > > +
> > > + smp_read_barrier_depends(); /* Dereferences after fetch of "t". */
> > > + return t;
> > > +}
> > > +#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > > index ad2a8df43757..6ad6af2ab028 100644
> > > --- a/kernel/rcu/update.c
> > > +++ b/kernel/rcu/update.c
> > > @@ -481,6 +481,28 @@ static void check_holdout_task(struct task_struct *t,
> > > sched_show_task(current);
> > > }
> > >
> > > +/* Check for nohz_full CPUs executing in userspace. */
> > > +static void check_no_hz_full_tasks(void)
> > > +{
> > > +#ifdef CONFIG_NO_HZ_FULL
> > > + int cpu;
> > > + struct task_struct *t;
> > > +
> > > + for_each_online_cpu(cpu) {
> > > + cond_resched();
> > > + rcu_read_lock();
> > > + t = rcu_dynticks_task_cur(cpu);
> > > + if (t == NULL || is_idle_task(t)) {
> > > + rcu_read_unlock();
> > > + continue;
> > > + }
> > > + if (ACCESS_ONCE(t->rcu_tasks_holdout))
> > > + ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> > > + rcu_read_unlock();
> > > + }
> > > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > > +}
> > > +
> > > /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
> > > static int __noreturn rcu_tasks_kthread(void *arg)
> > > {
> > > @@ -584,6 +606,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> > > lastreport = jiffies;
> > > firstreport = true;
> > > WARN_ON(signal_pending(current));
> > > + check_no_hz_full_tasks();
> > > rcu_read_lock();
> > > list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
> > > rcu_tasks_holdout_list)
> > >
> > > .
> > >
> >