Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751661Ab3IXMip (ORCPT ); Tue, 24 Sep 2013 08:38:45 -0400 Received: from merlin.infradead.org ([205.233.59.134]:46700 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750721Ab3IXMio (ORCPT ); Tue, 24 Sep 2013 08:38:44 -0400 Date: Tue, 24 Sep 2013 14:38:21 +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: <20130924123821.GT12926@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> <20130923175052.GA20991@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130923175052.GA20991@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: 7761 Lines: 288 OK, so another attempt. This one is actually fair in that it immediately forces a reader quiescent state by explicitly implementing reader-reader recursion. This does away with the potentially long pending writer case and can thus use the simpler global state. I don't really like this lock being fair, but alas. Also, please have a look at the atomic_dec_and_test(cpuhp_waitcount) and cpu_hotplug_done(). I think its ok, but I keep confusing myself. --- --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -16,6 +16,7 @@ #include #include #include +#include struct device; @@ -173,10 +174,49 @@ 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(); + + 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 +240,8 @@ 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 +49,115 @@ static int cpu_hotplug_disabled; #ifdef CONFIG_HOTPLUG_CPU -static struct { - struct task_struct *active_writer; - struct mutex lock; /* Synchronizes accesses to refcount, */ - /* - * Also blocks the new readers during - * an ongoing cpu hotplug operation. - */ - int refcount; -} cpu_hotplug = { - .active_writer = NULL, - .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock), - .refcount = 0, -}; +static struct task_struct *cpuhp_writer_task = NULL; -void get_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); +int __cpuhp_writer; +EXPORT_SYMBOL_GPL(__cpuhp_writer); +DEFINE_PER_CPU(unsigned int, __cpuhp_refcount); +EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount); + +static atomic_t cpuhp_waitcount; +static atomic_t cpuhp_slowcount; +static DECLARE_WAIT_QUEUE_HEAD(cpuhp_wq); + +void cpu_hotplug_init_task(struct task_struct *p) +{ + p->cpuhp_ref = 0; } -EXPORT_SYMBOL_GPL(get_online_cpus); -void put_online_cpus(void) +#define cpuhp_writer_wake() \ + wake_up_process(cpuhp_writer_task) + +#define cpuhp_writer_wait(cond) \ +do { \ + for (;;) { \ + set_current_state(TASK_UNINTERRUPTIBLE); \ + if (cond) \ + break; \ + schedule(); \ + } \ + __set_current_state(TASK_RUNNING); \ +} while (0) + +void __get_online_cpus(void) { - if (cpu_hotplug.active_writer == current) + if (cpuhp_writer_task == current) return; - mutex_lock(&cpu_hotplug.lock); - if (WARN_ON(!cpu_hotplug.refcount)) - cpu_hotplug.refcount++; /* try to fix things up */ + atomic_inc(&cpuhp_waitcount); + + /* + * We either call schedule() in the wait, or we'll fall through + * and reschedule on the preempt_enable() in get_online_cpus(). + */ + preempt_enable_no_resched(); + wait_event(cpuhp_wq, !__cpuhp_writer); + preempt_disable(); + + /* + * It would be possible for cpu_hotplug_done() to complete before + * the atomic_inc() above; in which case there is no writer waiting + * and doing a wakeup would be BAD (tm). + * + * If however we still observe cpuhp_writer_task here we know + * cpu_hotplug_done() is currently stuck waiting for cpuhp_waitcount. + */ + if (atomic_dec_and_test(&cpuhp_waitcount) && cpuhp_writer_task) + cpuhp_writer_wake(); +} +EXPORT_SYMBOL_GPL(__get_online_cpus); - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) - wake_up_process(cpu_hotplug.active_writer); - mutex_unlock(&cpu_hotplug.lock); +void __put_online_cpus(void) +{ + if (cpuhp_writer_task == current) + return; + if (atomic_dec_and_test(&cpuhp_slowcount)) + cpuhp_writer_wake(); } -EXPORT_SYMBOL_GPL(put_online_cpus); +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; + + lockdep_assert_held(&cpu_add_remove_lock); - for (;;) { - mutex_lock(&cpu_hotplug.lock); - if (likely(!cpu_hotplug.refcount)) - break; - __set_current_state(TASK_UNINTERRUPTIBLE); - mutex_unlock(&cpu_hotplug.lock); - schedule(); + __cpuhp_writer = 1; + cpuhp_writer_task = current; + + /* After this everybody will observe writer and take the slow path. */ + synchronize_sched(); + + /* Collapse the per-cpu refcount into slowcount */ + for_each_possible_cpu(cpu) { + count += per_cpu(__cpuhp_refcount, cpu); + per_cpu(__cpuhp_refcount, cpu) = 0; } + atomic_add(count, &cpuhp_slowcount); + + /* Wait for all readers to go away */ + cpuhp_writer_wait(!atomic_read(&cpuhp_slowcount)); } void cpu_hotplug_done(void) { - cpu_hotplug.active_writer = NULL; - mutex_unlock(&cpu_hotplug.lock); + /* Signal the writer is done */ + cpuhp_writer = 0; + wake_up_all(&cpuhp_wq); + + /* Wait for any pending readers to be running */ + cpuhp_writer_wait(!atomic_read(&cpuhp_waitcount)); + cpuhp_writer_task = NULL; } /* --- 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/