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Message-ID: <20140727184004.GR11241@linux.vnet.ibm.com> Date: Sun, 27 Jul 2014 11:40:05 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Pranith Kumar <bobby.prani@...il.com> Cc: 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, Frédéric Weisbecker <fweisbec@...il.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 Sat, Jul 26, 2014 at 09:34:11PM -0400, Pranith Kumar wrote: > On Fri, Jul 25, 2014 at 7:36 PM, Paul E. McKenney > <paulmck@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com> > > Please note one change below, with that > > Reviewed-by: Pranith Kumar <bobby.prani@...il.com> Good catch, fixed and applied your Reviewed-by. Thanx, Paul > > 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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