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    SubjectRe: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading callbacks unless requested
    On Mon, Jul 28, 2014 at 07:35:55PM -0400, Pranith Kumar wrote:
    > On Mon, Jul 28, 2014 at 7:17 PM, Paul E. McKenney
    > <paulmck@linux.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@linux.vnet.ibm.com>
    > > Reviewed-by: Pranith Kumar <bobby.prani@gmail.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.

    Good point, fixed.

    Thanx, Paul



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