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Date: Mon, 28 Jul 2014 19:35:55 -0400
From: Pranith Kumar <bobby.prani@...il.com>
To: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Lai Jiangshan <laijs@...fujitsu.com>,
Dipankar Sarma <dipankar@...ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Josh Triplett <josh@...htriplett.org>, tglx@...utronix.de,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
David Howells <dhowells@...hat.com>,
Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
Oleg Nesterov <oleg@...hat.com>,
Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading
callbacks unless requested
On Mon, Jul 28, 2014 at 7:17 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Mon, Jul 28, 2014 at 11:03:06PM +0200, Frederic Weisbecker wrote:
>>
>> Paul what do you think? If we keep that behaviour, Maybe you could blindly do
>> rcu_nocb_mask |= tick_nohz_full and remove the CONFIG_NO_HZ_FULL_ALL dependency
>> on RCU_NOCB_ALL?
>
> You mean something like the following updated patch?
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Fix attempt to avoid unsolicited offloading of callbacks
>
> 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@...ux.vnet.ibm.com>
> Reviewed-by: Pranith Kumar <bobby.prani@...il.com>
>
> 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/Kconfig b/init/Kconfig
> index 41066e49e880..6944acd00dff 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -737,7 +737,7 @@ choice
>
> config RCU_NOCB_CPU_NONE
> bool "No build_forced no-CBs CPUs"
> - depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
> + depends on RCU_NOCB_CPU
> help
> This option does not force any of the CPUs to be no-CBs CPUs.
> Only CPUs designated by the rcu_nocbs= boot parameter will be
> @@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE
>
> config RCU_NOCB_CPU_ZERO
> bool "CPU 0 is a build_forced no-CBs CPU"
> - depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
> + depends on RCU_NOCB_CPU
> help
> This option forces CPU 0 to be a no-CBs CPU, so that its RCU
> callbacks are invoked by a per-CPU kthread whose name begins
> 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..91208497e407 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 +2424,67 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
> trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
> }
>
> +void __init 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 you are removing the dependency on NO_HZ_FULL_ALL, I think this can just be
#ifdef CONFIG_NO_HZ_FULL
Otherwise, a kernel config with CONFIG_RCU_NOCB_NONE=y,
CONFIG_NO_HZ_FULL_ALL=y with nohz_full= boot parameter will not have a
rcu_nocb_mask.
--
Pranith
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