Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading callbacks unless requested

[Pranith Kumar]

On Fri, Jul 25, 2014 at 7:36 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
>         tick_nohz_full_mask when nohz_full= missing) in -tip
>         or -rcu.  To make this work on top of rcu/next, move the
>         call to rcu_organize_nocb_kthreads(rsp) to the end of the
>         for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
>
> Commit b58cc46c5f6b (rcu: Don't offload callbacks unless specifically
> requested) failed to adjust the callback lists of the CPUs that are
> known to be no-CBs CPUs only because they are also nohz_full= CPUs.
> This failure can result in callbacks that are posted during early boot
> getting stranded on nxtlist for CPUs whose no-CBs property becomes
> apparent late, and there can also be spurious warnings about offline
> CPUs posting callbacks.
>
> This commit fixes these problems by adding an early-boot rcu_init_nohz()
> that properly initializes the no-CBs CPUs.
>
> Note that kernels built with CONFIG_RCU_NOCB_CPU_ALL=y or with
> CONFIG_RCU_NOCB_CPU=n do not exhibit this bug.  Neither do kernels
> booted without the nohz_full= boot parameter.
>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>


Please note one change below, with that

Reviewed-by: Pranith Kumar <bobby.prani@xxxxxxxxx>


>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d231aa17b1d7..cc7bed1c90dc 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -269,6 +269,14 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
>                                          struct task_struct *next) { }
>  #endif /* CONFIG_RCU_USER_QS */
>
> +#ifdef CONFIG_RCU_NOCB_CPU
> +void rcu_init_nohz(void);
> +#else /* #ifdef CONFIG_RCU_NOCB_CPU */
> +static inline void rcu_init_nohz(void)
> +{
> +}
> +#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> +
>  /**
>   * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
>   * @a: Code that RCU needs to pay attention to.
> diff --git a/init/main.c b/init/main.c
> index e8ae1fef0908..5d8c83ae6c55 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -577,6 +577,7 @@ asmlinkage __visible void __init start_kernel(void)
>         idr_init_cache();
>         rcu_init();
>         tick_nohz_init();
> +       rcu_init_nohz();
>         context_tracking_init();
>         radix_tree_init();
>         /* init some links before init_ISA_irqs() */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 00dc411e9676..095d6e4d2fd7 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -85,33 +85,6 @@ static void __init rcu_bootup_announce_oddness(void)
>                 pr_info("\tBoot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
>         if (nr_cpu_ids != NR_CPUS)
>                 pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
> -#ifdef CONFIG_RCU_NOCB_CPU
> -#ifndef CONFIG_RCU_NOCB_CPU_NONE
> -       if (!have_rcu_nocb_mask) {
> -               zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
> -               have_rcu_nocb_mask = true;
> -       }
> -#ifdef CONFIG_RCU_NOCB_CPU_ZERO
> -       pr_info("\tOffload RCU callbacks from CPU 0\n");
> -       cpumask_set_cpu(0, rcu_nocb_mask);
> -#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
> -#ifdef CONFIG_RCU_NOCB_CPU_ALL
> -       pr_info("\tOffload RCU callbacks from all CPUs\n");
> -       cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
> -#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> -#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
> -       if (have_rcu_nocb_mask) {
> -               if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> -                       pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
> -                       cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> -                                   rcu_nocb_mask);
> -               }
> -               cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
> -               pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
> -               if (rcu_nocb_poll)
> -                       pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
> -       }
> -#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
>  }
>
>  #ifdef CONFIG_TREE_PREEMPT_RCU
> @@ -2451,6 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
>         trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
>  }
>
> +void rcu_init_nohz(void)
> +{
> +       int cpu;
> +       bool need_rcu_nocb_mask = true;
> +       struct rcu_state *rsp;
> +
> +#ifdef CONFIG_RCU_NOCB_CPU_NONE
> +       need_rcu_nocb_mask = false;
> +#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
> +
> +#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
> +       if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
> +               need_rcu_nocb_mask = true;
> +#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
> +
> +       if (!have_rcu_nocb_mask && need_rcu_nocb_mask) {
> +               zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
> +               have_rcu_nocb_mask = true;
> +       }
> +       if (!have_rcu_nocb_mask)
> +               return;
> +
> +#ifdef CONFIG_RCU_NOCB_CPU_ZERO
> +       pr_info("\tOffload RCU callbacks from CPU 0\n");
> +       cpumask_set_cpu(0, rcu_nocb_mask);
> +#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
> +#ifdef CONFIG_RCU_NOCB_CPU_ALL
> +       pr_info("\tOffload RCU callbacks from all CPUs\n");
> +       cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
> +#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> +#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
> +       cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> +#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */

I think we can still come here when there is garbage in
tick_nohz_full_mask. So checking again for tick_nohz_full_running is
necessary before cpumask_or().

> +
> +       if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> +               pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
> +               cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> +                           rcu_nocb_mask);
> +       }
> +       cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
> +       pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
> +       if (rcu_nocb_poll)
> +               pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
> +
> +       for_each_rcu_flavor(rsp) {
> +               for_each_cpu(cpu, rcu_nocb_mask) {
> +                       struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
> +
> +                       /*
> +                        * If there are early callbacks, they will need
> +                        * to be moved to the nocb lists.
> +                        */
> +                       WARN_ON_ONCE(rdp->nxttail[RCU_NEXT_TAIL] !=
> +                                    &rdp->nxtlist &&
> +                                    rdp->nxttail[RCU_NEXT_TAIL] != NULL);
> +                       init_nocb_callback_list(rdp);
> +               }
> +       }
> +}
> +
>  /* Initialize per-rcu_data variables for no-CBs CPUs. */
>  static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
>  {
> @@ -2479,10 +2512,6 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
>
>         if (rcu_nocb_mask == NULL)
>                 return;
> -#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
> -       if (tick_nohz_full_running)
> -               cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
>         if (ls == -1) {
>                 ls = int_sqrt(nr_cpu_ids);
>                 rcu_nocb_leader_stride = ls;
>



-- 
Pranith
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