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

[Pranith Kumar]

On Mon, Aug 11, 2014 at 6:48 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxxx>

Something about 3 minutes and 10 minutes is mixed up here!

> ---
>  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
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