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Message-ID: <20130212000100.GB2666@linux.vnet.ibm.com>
Date: Mon, 11 Feb 2013 16:01:01 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Rusty Russell <rusty@...tcorp.com.au>,
"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
Arjan van de Veen <arjan@...radead.org>,
Paul Turner <pjt@...gle.com>,
Richard Weinberger <rw@...utronix.de>,
Magnus Damm <magnus.damm@...il.com>
Subject: Re: [patch 32/40] rcu: Convert rcutree to hotplug state machine
On Thu, Jan 31, 2013 at 12:11:38PM -0000, Thomas Gleixner wrote:
> Do we really need so many states here ?
Well, all that RCU does for CPU_DYING is to do tracing, which could be
ditched. Required changes called out inline below.
All that the CPU_ONLINE and CPU_DOWN_PREPARE notifiers do is set
up affinity for the RCU-boost kthreads. These are unfortunately not
per-CPU kthreads, but perhaps something similar could be set up. This is
strictly a performance optimization, so the CPU_ONLINE notifier could
be replaced by having the kthread check which of its CPUs was online.
Unfortunately, the same is not true of CPU_DOWN_PREPARE because if the
kthread was too slow about it, the scheduler would get annoyed about a
kthread being runnable only on offlined CPUs.
It is not clear that this is worthwhile. Thoughts on other ways to
get this done?
Thanx, Paul
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> include/linux/cpuhotplug.h | 18 ++++++++
> kernel/cpu.c | 12 +++++
> kernel/rcutree.c | 95 ++++++++++++++++++++-------------------------
> 3 files changed, 73 insertions(+), 52 deletions(-)
>
> Index: linux-2.6/include/linux/cpuhotplug.h
> ===================================================================
> --- linux-2.6.orig/include/linux/cpuhotplug.h
> +++ linux-2.6/include/linux/cpuhotplug.h
> @@ -12,6 +12,7 @@ enum cpuhp_states {
> CPUHP_PERF_PREPARE,
> CPUHP_SCHED_MIGRATE_PREP,
> CPUHP_WORKQUEUE_PREP,
> + CPUHP_RCUTREE_PREPARE,
> CPUHP_NOTIFY_PREPARE,
> CPUHP_NOTIFY_DEAD,
> CPUHP_CPUFREQ_DEAD,
> @@ -27,6 +28,7 @@ enum cpuhp_states {
> CPUHP_AP_ARM64_TIMER_STARTING,
> CPUHP_AP_KVM_STARTING,
> CPUHP_AP_NOTIFY_DYING,
> + CPUHP_AP_RCUTREE_DYING,
Drop this.
> CPUHP_AP_X86_TBOOT_DYING,
> CPUHP_AP_S390_VTIME_DYING,
> CPUHP_AP_SCHED_NOHZ_DYING,
> @@ -39,6 +41,7 @@ enum cpuhp_states {
> CPUHP_SCHED_MIGRATE_ONLINE,
> CPUHP_WORKQUEUE_ONLINE,
> CPUHP_CPUFREQ_ONLINE,
> + CPUHP_RCUTREE_ONLINE,
> CPUHP_NOTIFY_ONLINE,
> CPUHP_NOTIFY_DOWN_PREPARE,
> CPUHP_PERF_X86_UNCORE_ONLINE,
> @@ -147,4 +150,19 @@ int workqueue_prepare_cpu(unsigned int c
> int workqueue_online_cpu(unsigned int cpu);
> int workqueue_offline_cpu(unsigned int cpu);
>
> +/* RCUtree hotplug events */
> +#if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
> +int rcutree_prepare_cpu(unsigned int cpu);
> +int rcutree_online_cpu(unsigned int cpu);
> +int rcutree_offline_cpu(unsigned int cpu);
> +int rcutree_dead_cpu(unsigned int cpu);
> +int rcutree_dying_cpu(unsigned int cpu);
And this...
> +#else
> +#define rcutree_prepare_cpu NULL
> +#define rcutree_online_cpu NULL
> +#define rcutree_offline_cpu NULL
> +#define rcutree_dead_cpu NULL
> +#define rcutree_dying_cpu NULL
And of course this.
> +#endif
> +
> #endif
> Index: linux-2.6/kernel/cpu.c
> ===================================================================
> --- linux-2.6.orig/kernel/cpu.c
> +++ linux-2.6/kernel/cpu.c
> @@ -755,6 +755,10 @@ static struct cpuhp_step cpuhp_bp_states
> .startup = workqueue_prepare_cpu,
> .teardown = NULL,
> },
> + [CPUHP_RCUTREE_PREPARE] = {
> + .startup = rcutree_prepare_cpu,
> + .teardown = rcutree_dead_cpu,
> + },
> [CPUHP_NOTIFY_PREPARE] = {
> .startup = notify_prepare,
> .teardown = NULL,
> @@ -787,6 +791,10 @@ static struct cpuhp_step cpuhp_bp_states
> .startup = workqueue_online_cpu,
> .teardown = workqueue_offline_cpu,
> },
> + [CPUHP_RCUTREE_ONLINE] = {
> + .startup = rcutree_online_cpu,
> + .teardown = rcutree_offline_cpu,
> + },
> [CPUHP_NOTIFY_ONLINE] = {
> .startup = notify_online,
> .teardown = NULL,
> @@ -813,6 +821,10 @@ static struct cpuhp_step cpuhp_ap_states
> .startup = NULL,
> .teardown = notify_dying,
> },
> + [CPUHP_AP_RCUTREE_DYING] = {
> + .startup = NULL,
> + .teardown = rcutree_dying_cpu,
> + },
> [CPUHP_AP_SCHED_NOHZ_DYING] = {
> .startup = NULL,
> .teardown = nohz_balance_exit_idle,
> Index: linux-2.6/kernel/rcutree.c
> ===================================================================
> --- linux-2.6.orig/kernel/rcutree.c
> +++ linux-2.6/kernel/rcutree.c
> @@ -2787,67 +2787,59 @@ rcu_init_percpu_data(int cpu, struct rcu
> mutex_unlock(&rsp->onoff_mutex);
> }
>
> -static void __cpuinit rcu_prepare_cpu(int cpu)
> +int __cpuinit rcutree_prepare_cpu(unsigned int cpu)
> {
> struct rcu_state *rsp;
>
> for_each_rcu_flavor(rsp)
> rcu_init_percpu_data(cpu, rsp,
> strcmp(rsp->name, "rcu_preempt") == 0);
> + rcu_prepare_kthreads(cpu);
> + return 0;
> }
>
> -/*
> - * Handle CPU online/offline notification events.
> - */
> -static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
> - unsigned long action, void *hcpu)
> +int __cpuinit rcutree_dead_cpu(unsigned int cpu)
> {
> - long cpu = (long)hcpu;
> - struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
> - struct rcu_node *rnp = rdp->mynode;
> struct rcu_state *rsp;
> - int ret = NOTIFY_OK;
>
> - trace_rcu_utilization("Start CPU hotplug");
> - switch (action) {
> - case CPU_UP_PREPARE:
> - case CPU_UP_PREPARE_FROZEN:
> - rcu_prepare_cpu(cpu);
> - rcu_prepare_kthreads(cpu);
> - break;
> - case CPU_ONLINE:
> - case CPU_DOWN_FAILED:
> - rcu_boost_kthread_setaffinity(rnp, -1);
> - break;
> - case CPU_DOWN_PREPARE:
> - if (nocb_cpu_expendable(cpu))
> - rcu_boost_kthread_setaffinity(rnp, cpu);
> - else
> - ret = NOTIFY_BAD;
> - break;
> - case CPU_DYING:
> - case CPU_DYING_FROZEN:
> - /*
> - * The whole machine is "stopped" except this CPU, so we can
> - * touch any data without introducing corruption. We send the
> - * dying CPU's callbacks to an arbitrarily chosen online CPU.
> - */
> - for_each_rcu_flavor(rsp)
> - rcu_cleanup_dying_cpu(rsp);
> - rcu_cleanup_after_idle(cpu);
> - break;
> - case CPU_DEAD:
> - case CPU_DEAD_FROZEN:
> - case CPU_UP_CANCELED:
> - case CPU_UP_CANCELED_FROZEN:
> - for_each_rcu_flavor(rsp)
> - rcu_cleanup_dead_cpu(cpu, rsp);
> - break;
> - default:
> - break;
> - }
> - trace_rcu_utilization("End CPU hotplug");
> - return ret;
> + for_each_rcu_flavor(rsp)
> + rcu_cleanup_dead_cpu(cpu, rsp);
> + return 0;
> +}
> +
> +static void __cpuinit rcutree_affinity_setting(unsigned int cpu, int outgoing)
> +{
> + struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
> +
> + rcu_boost_kthread_setaffinity(rdp->mynode, outgoing);
> +}
> +
> +int __cpuinit rcutree_online_cpu(unsigned int cpu)
> +{
> + rcutree_affinity_setting(cpu, -1);
> + return 0;
> +}
> +
> +int __cpuinit rcutree_offline_cpu(unsigned int cpu)
> +{
> + if (!nocb_cpu_expendable(cpu))
> + return -EINVAL;
> + rcutree_affinity_setting(cpu, cpu);
> + return 0;
> +}
> +
> +int __cpuinit rcutree_dying_cpu(unsigned int cpu)
> +{
> + struct rcu_state *rsp;
> + /*
> + * The whole machine is "stopped" except this CPU, so we can
> + * touch any data without introducing corruption. We send the
> + * dying CPU's callbacks to an arbitrarily chosen online CPU.
> + */
> + for_each_rcu_flavor(rsp)
> + rcu_cleanup_dying_cpu(rsp);
> + rcu_cleanup_after_idle(cpu);
> + return 0;
> }
And rcu_dying_cpu() above, along with both definitions of
rcu_cleanup_dying_cpu().
> /*
> @@ -3071,9 +3063,8 @@ void __init rcu_init(void)
> * this is called early in boot, before either interrupts
> * or the scheduler are operational.
> */
> - cpu_notifier(rcu_cpu_notify, 0);
> for_each_online_cpu(cpu)
> - rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
> + rcutree_prepare_cpu(cpu);
> check_cpu_stall_init();
> }
>
>
>
--
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