Re: [PATCH v5 tip/core/rcu 08/16] rcu: Add stall-warning checks for RCU-tasks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Aug 14, 2014 at 05:39:54PM -0400, Pranith Kumar wrote:
> On Mon, Aug 11, 2014 at 6:48 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> >
> > This commit adds a three-minute RCU-tasks stall warning.  The actual
> > time is controlled by the boot/sysfs parameter rcu_task_stall_timeout,
> > with values less than or equal to zero disabling the stall warnings.
> > The default value is three minutes, which means that the tasks that
> > have not yet responded will get their stacks dumped every ten minutes,
> > until they pass through a voluntary context switch.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Something about 3 minutes and 10 minutes is mixed up here!

Good catch, updated the commit log to also say ten minutes.

							Thanx, Paul

> > ---
> >  Documentation/kernel-parameters.txt |  5 +++++
> >  kernel/rcu/update.c                 | 27 ++++++++++++++++++++++++---
> >  2 files changed, 29 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> > index 910c3829f81d..8cdbde7b17f5 100644
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
> > @@ -2921,6 +2921,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >         rcupdate.rcu_cpu_stall_timeout= [KNL]
> >                         Set timeout for RCU CPU stall warning messages.
> >
> > +       rcupdate.rcu_task_stall_timeout= [KNL]
> > +                       Set timeout in jiffies for RCU task stall warning
> > +                       messages.  Disable with a value less than or equal
> > +                       to zero.
> > +
> >         rdinit=         [KNL]
> >                         Format: <full_path>
> >                         Run specified binary instead of /init from the ramdisk,
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > index 8f53a41dd9ee..f1535404a79e 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -374,7 +374,7 @@ static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
> >  DEFINE_SRCU(tasks_rcu_exit_srcu);
> >
> >  /* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
> > -static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 3;
> > +static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
> >  module_param(rcu_task_stall_timeout, int, 0644);
> >
> >  /* Post an RCU-tasks callback. */
> > @@ -449,7 +449,8 @@ void rcu_barrier_tasks(void)
> >  EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
> >
> >  /* See if tasks are still holding out, complain if so. */
> > -static void check_holdout_task(struct task_struct *t)
> > +static void check_holdout_task(struct task_struct *t,
> > +                              bool needreport, bool *firstreport)
> >  {
> >         if (!ACCESS_ONCE(t->rcu_tasks_holdout) ||
> >             t->rcu_tasks_nvcsw != ACCESS_ONCE(t->nvcsw) ||
> > @@ -457,7 +458,15 @@ static void check_holdout_task(struct task_struct *t)
> >                 ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> >                 list_del_rcu(&t->rcu_tasks_holdout_list);
> >                 put_task_struct(t);
> > +               return;
> >         }
> > +       if (!needreport)
> > +               return;
> > +       if (*firstreport) {
> > +               pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
> > +               *firstreport = false;
> > +       }
> > +       sched_show_task(t);
> >  }
> >
> >  /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
> > @@ -465,6 +474,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> >  {
> >         unsigned long flags;
> >         struct task_struct *g, *t;
> > +       unsigned long lastreport;
> >         struct rcu_head *list;
> >         struct rcu_head *next;
> >         LIST_HEAD(rcu_tasks_holdouts);
> > @@ -543,13 +553,24 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> >                  * of holdout tasks, removing any that are no longer
> >                  * holdouts.  When the list is empty, we are done.
> >                  */
> > +               lastreport = jiffies;
> >                 while (!list_empty(&rcu_tasks_holdouts)) {
> > +                       bool firstreport;
> > +                       bool needreport;
> > +                       int rtst;
> > +
> >                         schedule_timeout_interruptible(HZ);
> > +                       rtst = ACCESS_ONCE(rcu_task_stall_timeout);
> > +                       needreport = rtst > 0 &&
> > +                                    time_after(jiffies, lastreport + rtst);
> > +                       if (needreport)
> > +                               lastreport = jiffies;
> > +                       firstreport = true;
> >                         WARN_ON(signal_pending(current));
> >                         rcu_read_lock();
> >                         list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
> >                                                 rcu_tasks_holdout_list)
> > -                               check_holdout_task(t);
> > +                               check_holdout_task(t, needreport, &firstreport);
> >                         rcu_read_unlock();
> >                 }
> >
> > --
> > 1.8.1.5
> >
> 
> 
> 
> -- 
> Pranith
>