Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754602Ab3IXUYv (ORCPT ); Tue, 24 Sep 2013 16:24:51 -0400 Received: from merlin.infradead.org ([205.233.59.134]:58311 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753370Ab3IXUYu (ORCPT ); Tue, 24 Sep 2013 16:24:50 -0400 Date: Tue, 24 Sep 2013 22:24:23 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Paul McKenney , Thomas Gleixner , Steven Rostedt Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20130924202423.GW12926@twins.programming.kicks-ass.net> References: <1378805550-29949-1-git-send-email-mgorman@suse.de> <1378805550-29949-38-git-send-email-mgorman@suse.de> <20130917143003.GA29354@twins.programming.kicks-ass.net> <20130917162050.GK22421@suse.de> <20130917164505.GG12926@twins.programming.kicks-ass.net> <20130918154939.GZ26785@twins.programming.kicks-ass.net> <20130919143241.GB26785@twins.programming.kicks-ass.net> <20130921163404.GA8545@redhat.com> <20130923092955.GV9326@twins.programming.kicks-ass.net> <20130923173203.GA20392@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130923173203.GA20392@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8318 Lines: 331 On Mon, Sep 23, 2013 at 07:32:03PM +0200, Oleg Nesterov wrote: > > static void cpuhp_wait_refcount(void) > > { > > for (;;) { > > unsigned int rc1, rc2; > > > > rc1 = cpuhp_refcount(); > > set_current_state(TASK_UNINTERRUPTIBLE); /* MB */ > > rc2 = cpuhp_refcount(); > > > > if (rc1 == rc2 && !rc1) > > But this only makes the race above "theoretical ** 2". Both > cpuhp_refcount()'s can be equally fooled. > > Looks like, cpuhp_refcount() should take all per-cpu cpuhp_lock's > before it reads __cpuhp_refcount. Ah, so SRCU has a solution for this using a sequence count. So now we drop from a no memory barriers fast path, into a memory barrier 'slow' path into blocking. Only once we block do we hit global state.. --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -16,6 +16,7 @@ #include #include #include +#include struct device; @@ -173,10 +174,50 @@ extern struct bus_type cpu_subsys; #ifdef CONFIG_HOTPLUG_CPU /* Stop CPUs going up and down. */ +extern void cpu_hotplug_init_task(struct task_struct *p); + extern void cpu_hotplug_begin(void); extern void cpu_hotplug_done(void); -extern void get_online_cpus(void); -extern void put_online_cpus(void); + +extern int __cpuhp_writer; +DECLARE_PER_CPU(unsigned int, __cpuhp_refcount); + +extern void __get_online_cpus(void); + +static inline void get_online_cpus(void) +{ + might_sleep(); + + /* Support reader-in-reader recursion */ + if (current->cpuhp_ref++) { + barrier(); + return; + } + + preempt_disable(); + if (likely(!__cpuhp_writer)) + __this_cpu_inc(__cpuhp_refcount); + else + __get_online_cpus(); + preempt_enable(); +} + +extern void __put_online_cpus(void); + +static inline void put_online_cpus(void) +{ + barrier(); + if (--current->cpuhp_ref) + return; + + preempt_disable(); + if (likely(!__cpuhp_writer)) + __this_cpu_dec(__cpuhp_refcount); + else + __put_online_cpus(); + preempt_enable(); +} + extern void cpu_hotplug_disable(void); extern void cpu_hotplug_enable(void); #define hotcpu_notifier(fn, pri) cpu_notifier(fn, pri) @@ -200,6 +241,8 @@ static inline void cpu_hotplug_driver_un #else /* CONFIG_HOTPLUG_CPU */ +static inline void cpu_hotplug_init_task(struct task_struct *p) {} + static inline void cpu_hotplug_begin(void) {} static inline void cpu_hotplug_done(void) {} #define get_online_cpus() do { } while (0) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1454,6 +1454,9 @@ struct task_struct { unsigned int sequential_io; unsigned int sequential_io_avg; #endif +#ifdef CONFIG_HOTPLUG_CPU + int cpuhp_ref; +#endif }; /* Future-safe accessor for struct task_struct's cpus_allowed. */ --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -49,88 +49,148 @@ static int cpu_hotplug_disabled; #ifdef CONFIG_HOTPLUG_CPU -static struct { - struct task_struct *active_writer; - struct mutex lock; /* Synchronizes accesses to refcount, */ +int __cpuhp_writer; +EXPORT_SYMBOL_GPL(__cpuhp_writer); + +DEFINE_PER_CPU(unsigned int, __cpuhp_refcount); +EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount); + +static DEFINE_PER_CPU(unsigned int, cpuhp_seq); +static atomic_t cpuhp_waitcount; +static DECLARE_WAIT_QUEUE_HEAD(cpuhp_readers); +static DECLARE_WAIT_QUEUE_HEAD(cpuhp_writer); + +void cpu_hotplug_init_task(struct task_struct *p) +{ + p->cpuhp_ref = 0; +} + +void __get_online_cpus(void) +{ + if (__cpuhp_writer == 1) { + /* See __srcu_read_lock() */ + __this_cpu_inc(__cpuhp_refcount); + smp_mb(); + __this_cpu_inc(cpuhp_seq); + return; + } + + atomic_inc(&cpuhp_waitcount); + /* - * Also blocks the new readers during - * an ongoing cpu hotplug operation. + * We either call schedule() in the wait, or we'll fall through + * and reschedule on the preempt_enable() in get_online_cpus(). */ - int refcount; -} cpu_hotplug = { - .active_writer = NULL, - .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock), - .refcount = 0, -}; + preempt_enable_no_resched(); + wait_event(cpuhp_readers, !__cpuhp_writer); + preempt_disable(); -void get_online_cpus(void) + /* + * XXX list_empty_careful(&cpuhp_readers.task_list) ? + */ + if (atomic_dec_and_test(&cpuhp_waitcount)) + wake_up_all(&cpuhp_writer); +} +EXPORT_SYMBOL_GPL(__get_online_cpus); + +void __put_online_cpus(void) { - might_sleep(); - if (cpu_hotplug.active_writer == current) - return; - mutex_lock(&cpu_hotplug.lock); - cpu_hotplug.refcount++; - mutex_unlock(&cpu_hotplug.lock); + /* See __srcu_read_unlock() */ + smp_mb(); + this_cpu_dec(__cpuhp_refcount); + /* Prod writer to recheck readers_active */ + wake_up_all(&cpuhp_writer); } -EXPORT_SYMBOL_GPL(get_online_cpus); +EXPORT_SYMBOL_GPL(__put_online_cpus); -void put_online_cpus(void) +static unsigned int cpuhp_seq(void) { - if (cpu_hotplug.active_writer == current) - return; - mutex_lock(&cpu_hotplug.lock); + unsigned int seq = 0; + int cpu; - if (WARN_ON(!cpu_hotplug.refcount)) - cpu_hotplug.refcount++; /* try to fix things up */ + for_each_possible_cpu(cpu) + seq += per_cpu(cpuhp_seq, cpu); - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) - wake_up_process(cpu_hotplug.active_writer); - mutex_unlock(&cpu_hotplug.lock); + return seq; +} + +static unsigned int cpuhp_refcount(void) +{ + unsigned int refcount = 0; + int cpu; + for_each_possible_cpu(cpu) + refcount += per_cpu(__cpuhp_refcount, cpu); + + return refcount; +} + +/* + * See srcu_readers_active_idx_check() + */ +static bool cpuhp_readers_active_check(void) +{ + unsigned int seq = cpuhp_seq(); + + smp_mb(); + + if (cpuhp_refcount() != 0) + return false; + + smp_mb(); + + return cpuhp_seq() == seq; } -EXPORT_SYMBOL_GPL(put_online_cpus); /* * This ensures that the hotplug operation can begin only when the * refcount goes to zero. * - * Note that during a cpu-hotplug operation, the new readers, if any, - * will be blocked by the cpu_hotplug.lock - * * Since cpu_hotplug_begin() is always called after invoking * cpu_maps_update_begin(), we can be sure that only one writer is active. - * - * Note that theoretically, there is a possibility of a livelock: - * - Refcount goes to zero, last reader wakes up the sleeping - * writer. - * - Last reader unlocks the cpu_hotplug.lock. - * - A new reader arrives at this moment, bumps up the refcount. - * - The writer acquires the cpu_hotplug.lock finds the refcount - * non zero and goes to sleep again. - * - * However, this is very difficult to achieve in practice since - * get_online_cpus() not an api which is called all that often. - * */ void cpu_hotplug_begin(void) { - cpu_hotplug.active_writer = current; + unsigned int count = 0; + int cpu; - for (;;) { - mutex_lock(&cpu_hotplug.lock); - if (likely(!cpu_hotplug.refcount)) - break; - __set_current_state(TASK_UNINTERRUPTIBLE); - mutex_unlock(&cpu_hotplug.lock); - schedule(); - } + lockdep_assert_held(&cpu_add_remove_lock); + + /* allow reader-in-writer recursion */ + current->cpuhp_ref++; + + /* make readers take the slow path */ + __cpuhp_writer = 1; + + /* See percpu_down_write() */ + synchronize_sched(); + + /* make readers block */ + __cpuhp_writer = 2; + + /* Wait for all readers to go away */ + wait_event(cpuhp_writer, cpuhp_readers_active_check()); } void cpu_hotplug_done(void) { - cpu_hotplug.active_writer = NULL; - mutex_unlock(&cpu_hotplug.lock); + /* Signal the writer is done, no fast path yet */ + __cpuhp_writer = 1; + wake_up_all(&cpuhp_readers); + + /* See percpu_up_write() */ + synchronize_sched(); + + /* Let em rip */ + __cpuhp_writer = 0 + current->cpuhp_ref--; + + /* + * Wait for any pending readers to be running. This ensures readers + * after writer and avoids writers starving readers. + */ + wait_event(cpuhp_writer, !atomic_read(&cpuhp_waitcount)); } /* --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1736,6 +1736,8 @@ static void __sched_fork(unsigned long c INIT_LIST_HEAD(&p->numa_entry); p->numa_group = NULL; #endif /* CONFIG_NUMA_BALANCING */ + + cpu_hotplug_init_task(p); } #ifdef CONFIG_NUMA_BALANCING -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/