Date:	Thu, 3 Oct 2013 09:26:50 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] hotplug: Optimize {get,put}_online_cpus()

On Wed, Oct 02, 2013 at 04:56:56PM +0200, Peter Zijlstra wrote:
> The current implementation of get_online_cpus() is global of nature
> and thus not suited for any kind of common usage.
> 
> Re-implement the current recursive r/w cpu hotplug lock such that the
> read side locks are as light as possible.
> 
> The current cpu hotplug lock is entirely reader biased; but since
> readers are expensive there aren't a lot of them about and writer
> starvation isn't a particular problem.
> 
> However by making the reader side more usable there is a fair chance
> it will get used more and thus the starvation issue becomes a real
> possibility.
> 
> Therefore this new implementation is fair, alternating readers and
> writers; this however requires per-task state to allow the reader
> recursion -- this new task_struct member is placed in a 4 byte hole on
> 64bit builds.
> 
> Many comments are contributed by Paul McKenney, and many previous
> attempts were shown to be inadequate by both Paul and Oleg; many
> thanks to them for persisting to poke holes in my attempts.
> 
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>

One change to a now-obsolete comment called out below, with that
change:

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  include/linux/cpu.h   |   67 ++++++++++++++
>  include/linux/sched.h |    3 
>  kernel/cpu.c          |  226 ++++++++++++++++++++++++++++++++++++--------------
>  kernel/sched/core.c   |    2 
>  4 files changed, 235 insertions(+), 63 deletions(-)
> 
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -16,6 +16,8 @@
>  #include <linux/node.h>
>  #include <linux/compiler.h>
>  #include <linux/cpumask.h>
> +#include <linux/percpu.h>
> +#include <linux/sched.h>
> 
>  struct device;
> 
> @@ -173,10 +175,69 @@ 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_state;
> +DECLARE_PER_CPU(unsigned int, __cpuhp_refcount);
> +
> +extern void __get_online_cpus(void);
> +
> +static inline void get_online_cpus(void)
> +{
> +	might_sleep();
> +
> +	/* Support reader recursion */
> +	/* The value was >= 1 and remains so, reordering causes no harm. */
> +	if (current->cpuhp_ref++)
> +		return;
> +
> +	preempt_disable();
> +	/*
> +	 * We are in an RCU-sched read-side critical section, so the writer
> +	 * cannot both change __cpuhp_state from readers_fast and start
> +	 * checking counters while we are here. So if we see !__cpuhp_state,
> +	 * we know that the writer won't be checking until we past the
> +	 * preempt_enable() and that once the synchronize_sched() is done, the
> +	 * writer will see anything we did within this RCU-sched read-side
> +	 * critical section.
> +	 */
> +	if (likely(!__cpuhp_state))
> +		__this_cpu_inc(__cpuhp_refcount);
> +	else
> +		__get_online_cpus(); /* Unconditional memory barrier. */
> +	preempt_enable();
> +	/*
> +	 * The barrier() from preempt_enable() prevents the compiler from
> +	 * bleeding the critical section out.
> +	 */
> +}
> +
> +extern void __put_online_cpus(void);
> +
> +static inline void put_online_cpus(void)
> +{
> +	/* The value was >= 1 and remains so, reordering causes no harm. */
> +	if (--current->cpuhp_ref)
> +		return;
> +
> +	/*
> +	 * The barrier() in preempt_disable() prevents the compiler from
> +	 * bleeding the critical section out.
> +	 */
> +	preempt_disable();
> +	/*
> +	 * Same as in get_online_cpus().
> +	 */
> +	if (likely(!__cpuhp_state))
> +		__this_cpu_dec(__cpuhp_refcount);
> +	else
> +		__put_online_cpus(); /* Unconditional memory barrier. */
> +	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 +261,8 @@ 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
> @@ -1039,6 +1039,9 @@ struct task_struct {
>  #ifdef CONFIG_SMP
>  	struct llist_node wake_entry;
>  	int on_cpu;
> +#ifdef CONFIG_HOTPLUG_CPU
> +	int cpuhp_ref;
> +#endif
>  	struct task_struct *last_wakee;
>  	unsigned long wakee_flips;
>  	unsigned long wakee_flip_decay_ts;
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -49,88 +49,192 @@ 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,
> -};
> +enum { readers_fast = 0, readers_slow, readers_block };
> 
> -void get_online_cpus(void)
> +int __cpuhp_state;
> +EXPORT_SYMBOL_GPL(__cpuhp_state);
> +
> +DEFINE_PER_CPU(unsigned int, __cpuhp_refcount);
> +EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount);
> +
> +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)
>  {
> -	might_sleep();
> -	if (cpu_hotplug.active_writer == current)
> +	p->cpuhp_ref = 0;
> +}
> +
> +void __get_online_cpus(void)
> +{
> +again:
> +	__this_cpu_inc(__cpuhp_refcount);
> +
> +	/*
> +	 * Due to having preemption disabled the decrement happens on
> +	 * the same CPU as the increment, avoiding the
> +	 * increment-on-one-CPU-and-decrement-on-another problem.
> +	 *
> +	 * And yes, if the reader misses the writer's assignment of
> +	 * readers_block to __cpuhp_state, then the writer is
> +	 * guaranteed to see the reader's increment.  Conversely, any
> +	 * readers that increment their __cpuhp_refcount after the
> +	 * writer looks are guaranteed to see the readers_block value,
> +	 * which in turn means that they are guaranteed to immediately
> +	 * decrement their __cpuhp_refcount, so that it doesn't matter
> +	 * that the writer missed them.
> +	 */
> +
> +	smp_mb(); /* A matches D */
> +
> +	if (likely(__cpuhp_state != readers_block))
>  		return;
> -	mutex_lock(&cpu_hotplug.lock);
> -	cpu_hotplug.refcount++;
> -	mutex_unlock(&cpu_hotplug.lock);
> 
> +	/*
> +	 * Make sure an outgoing writer sees the waitcount to ensure we
> +	 * make progress.
> +	 */
> +	atomic_inc(&cpuhp_waitcount);
> +
> +	/*
> +	 * Per the above comment; we still have preemption disabled and
> +	 * will thus decrement on the same CPU as we incremented.
> +	 */
> +	__put_online_cpus();
> +
> +	/*
> +	 * 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_readers, __cpuhp_state != readers_block);
> +	preempt_disable();
> +
> +	/*
> +	 * Given we've still got preempt_disabled and new cpu_hotplug_begin()
> +	 * must do a synchronize_sched() we're guaranteed a successfull
> +	 * acquisition this time -- even if we wake the current
> +	 * cpu_hotplug_end() now.
> +	 */
> +	if (atomic_dec_and_test(&cpuhp_waitcount))
> +		wake_up(&cpuhp_writer);
> +
> +	goto again:
>  }
> -EXPORT_SYMBOL_GPL(get_online_cpus);
> +EXPORT_SYMBOL_GPL(__get_online_cpus);
> 
> -void put_online_cpus(void)
> +void __put_online_cpus(void)
>  {
> -	if (cpu_hotplug.active_writer == current)
> -		return;
> -	mutex_lock(&cpu_hotplug.lock);
> +	smp_mb(); /* B matches C */
> +	/*
> +	 * In other words, if they see our decrement (presumably to aggregate
> +	 * zero, as that is the only time it matters) they will also see our
> +	 * critical section.
> +	 */
> +	this_cpu_dec(__cpuhp_refcount);
> +
> +	/* Prod writer to recheck readers_active */
> +	wake_up(&cpuhp_writer);
> +}
> +EXPORT_SYMBOL_GPL(__put_online_cpus);
> +
> +#define per_cpu_sum(var)						\
> +({ 									\
> + 	typeof(var) __sum = 0;						\
> + 	int cpu;							\
> + 	for_each_possible_cpu(cpu)					\
> + 		__sum += per_cpu(var, cpu);				\
> + 	__sum;								\
> +)}
> 
> -	if (WARN_ON(!cpu_hotplug.refcount))
> -		cpu_hotplug.refcount++; /* try to fix things up */
> +/*
> + * See srcu_readers_active_idx_check() for a rather more detailed explanation.
> + */

The above comment is now obsolete, suggest something like:

/*
 * Return true if the modular sum of the __cpuhp_refcount per-CPU
 * variables is zero.  If this sum is zero, then it is stable
 * due to the fact that if any newly arriving readers increment
 * a given counter, they will immediately decrement that same
 * counter.
 */

> +static bool cpuhp_readers_active_check(void)
> +{
> +	if (per_cpu_sum(__cpuhp_refcount) != 0)
> +		return false;
> 
> -	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
> -		wake_up_process(cpu_hotplug.active_writer);
> -	mutex_unlock(&cpu_hotplug.lock);
> +	/*
> +	 * If we observed the decrement; ensure we see the entire critical
> +	 * section.
> +	 */
> +
> +	smp_mb(); /* C matches B */
> 
> +	return true;
>  }
> -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.
> - *
> + * This will notify new readers to block and wait for all active readers to
> + * complete.
>   */
>  void cpu_hotplug_begin(void)
>  {
> -	cpu_hotplug.active_writer = current;
> +	/*
> +	 * Since cpu_hotplug_begin() is always called after invoking
> +	 * cpu_maps_update_begin(), we can be sure that only one writer is
> +	 * active.
> +	 */
> +	lockdep_assert_held(&cpu_add_remove_lock);
> +
> +	/* Allow reader-in-writer recursion. */
> +	current->cpuhp_ref++;
> +
> +	/* Notify readers to take the slow path. */
> +	__cpuhp_state = readers_slow;
> +
> +	/* See percpu_down_write(); guarantees all readers take the slow path */
> +	synchronize_sched();
> +
> +	/*
> +	 * Notify new readers to block; up until now, and thus throughout the
> +	 * longish synchronize_sched() above, new readers could still come in.
> +	 */
> +	__cpuhp_state = readers_block;
> 
> -	for (;;) {
> -		mutex_lock(&cpu_hotplug.lock);
> -		if (likely(!cpu_hotplug.refcount))
> -			break;
> -		__set_current_state(TASK_UNINTERRUPTIBLE);
> -		mutex_unlock(&cpu_hotplug.lock);
> -		schedule();
> -	}
> +	smp_mb(); /* D matches A */
> +
> +	/*
> +	 * If they don't see our writer of readers_block to __cpuhp_state,
> +	 * then we are guaranteed to see their __cpuhp_refcount increment, and
> +	 * therefore will wait for them.
> +	 */
> +
> +	/* Wait for all now active readers to complete. */
> +	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.
> +	 *
> +	 * One reason that we cannot just immediately flip to readers_fast is
> +	 * that new readers might fail to see the results of this writer's
> +	 * critical section.
> +	 */
> +	__cpuhp_state = readers_slow;
> +	wake_up_all(&cpuhp_readers);
> +
> +	/*
> +	 * The wait_event()/wake_up_all() prevents the race where the readers
> +	 * are delayed between fetching __cpuhp_state and blocking.
> +	 */
> +
> +	/* See percpu_up_write(); readers will no longer attempt to block. */
> +	synchronize_sched();
> +
> +	/* Let 'em rip */
> +	__cpuhp_state = readers_fast;
> +	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
> @@ -1635,6 +1635,8 @@ static void __sched_fork(struct task_str
>  	p->numa_scan_period = sysctl_numa_balancing_scan_delay;
>  	p->numa_work.next = &p->numa_work;
>  #endif /* CONFIG_NUMA_BALANCING */
> +
> +	cpu_hotplug_init_task(p);
>  }
> 
>  #ifdef CONFIG_NUMA_BALANCING
> 
> 

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