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

From: Pranith Kumar
Date: Sat Jul 26 2014 - 21:35:00 EST


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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