Message-ID: <20121101154027.GC3888@suse.de>
Date:	Thu, 1 Nov 2012 15:40:28 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 26/31] sched, numa, mm: Add fault driven placement and
 migration policy

On Thu, Oct 25, 2012 at 02:16:43PM +0200, Peter Zijlstra wrote:
> As per the problem/design document Documentation/scheduler/numa-problem.txt
> implement 3ac & 4.
> 
> ( A pure 3a was found too unstable, I did briefly try 3bc
>   but found no significant improvement. )
> 
> Implement a per-task memory placement scheme relying on a regular
> PROT_NONE 'migration' fault to scan the memory space of the procress
> and uses a two stage migration scheme to reduce the invluence of
> unlikely usage relations.
> 
> It relies on the assumption that the compute part is tied to a
> paticular task and builds a task<->page relation set to model the
> compute<->data relation.
> 
> In the previous patch we made memory migrate towards where the task
> is running, here we select the node on which most memory is located
> as the preferred node to run on.
> 
> This creates a feed-back control loop between trying to schedule a
> task on a node and migrating memory towards the node the task is
> scheduled on. 
> 

Ok.

> Suggested-by: Andrea Arcangeli <aarcange@...hat.com>
> Suggested-by: Rik van Riel <riel@...hat.com>
> Fixes-by: David Rientjes <rientjes@...gle.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  include/linux/mm_types.h |    4 +
>  include/linux/sched.h    |   35 +++++++--
>  kernel/sched/core.c      |   16 ++++
>  kernel/sched/fair.c      |  175 +++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/features.h  |    1 
>  kernel/sched/sched.h     |   31 +++++---
>  kernel/sysctl.c          |   31 +++++++-
>  mm/huge_memory.c         |    7 +
>  mm/memory.c              |    4 -
>  9 files changed, 282 insertions(+), 22 deletions(-)
> Index: tip/include/linux/mm_types.h
> ===================================================================
> --- tip.orig/include/linux/mm_types.h
> +++ tip/include/linux/mm_types.h
> @@ -403,6 +403,10 @@ struct mm_struct {
>  #ifdef CONFIG_CPUMASK_OFFSTACK
>  	struct cpumask cpumask_allocation;
>  #endif
> +#ifdef CONFIG_SCHED_NUMA
> +	unsigned long numa_next_scan;

comment. 

> +	int numa_scan_seq;

comment! at least the other one is easy to guess. This thing looks like
it's preventing multiple threads in a process space scanning and
updating PTEs at the same time. Effectively it's a type of barrier but
without a comment I'm not sure if what it's doing is what you expect it
to be doing or something else entirely.

> +#endif
>  	struct uprobes_state uprobes_state;
>  };
>  
> Index: tip/include/linux/sched.h
> ===================================================================
> --- tip.orig/include/linux/sched.h
> +++ tip/include/linux/sched.h
> @@ -1481,9 +1481,16 @@ struct task_struct {
>  	short pref_node_fork;
>  #endif
>  #ifdef CONFIG_SCHED_NUMA
> -	int node;
> +	int node;			/* task home node   */
> +	int numa_scan_seq;
> +	int numa_migrate_seq;
> +	unsigned int numa_scan_period;
> +	u64 node_stamp;			/* migration stamp  */
>  	unsigned long numa_contrib;
> -#endif
> +	unsigned long *numa_faults;
> +	struct callback_head numa_work;
> +#endif /* CONFIG_SCHED_NUMA */
> +
>  	struct rcu_head rcu;
>  
>  	/*
> @@ -1558,15 +1565,24 @@ struct task_struct {
>  /* Future-safe accessor for struct task_struct's cpus_allowed. */
>  #define tsk_cpus_allowed(tsk) (&(tsk)->cpus_allowed)
>  
> +#ifdef CONFIG_SCHED_NUMA
>  static inline int tsk_home_node(struct task_struct *p)
>  {
> -#ifdef CONFIG_SCHED_NUMA
>  	return p->node;
> +}
> +
> +extern void task_numa_fault(int node, int pages);
>  #else
> +static inline int tsk_home_node(struct task_struct *p)
> +{
>  	return -1;
> -#endif
>  }
>  
> +static inline void task_numa_fault(int node, int pages)
> +{
> +}
> +#endif /* CONFIG_SCHED_NUMA */
> +
>  /*
>   * Priority of a process goes from 0..MAX_PRIO-1, valid RT
>   * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
> @@ -2004,6 +2020,10 @@ enum sched_tunable_scaling {
>  };
>  extern enum sched_tunable_scaling sysctl_sched_tunable_scaling;
>  
> +extern unsigned int sysctl_sched_numa_scan_period_min;
> +extern unsigned int sysctl_sched_numa_scan_period_max;
> +extern unsigned int sysctl_sched_numa_settle_count;
> +
>  #ifdef CONFIG_SCHED_DEBUG
>  extern unsigned int sysctl_sched_migration_cost;
>  extern unsigned int sysctl_sched_nr_migrate;
> @@ -2014,18 +2034,17 @@ extern unsigned int sysctl_sched_shares_
>  int sched_proc_update_handler(struct ctl_table *table, int write,
>  		void __user *buffer, size_t *length,
>  		loff_t *ppos);
> -#endif
> -#ifdef CONFIG_SCHED_DEBUG
> +
>  static inline unsigned int get_sysctl_timer_migration(void)
>  {
>  	return sysctl_timer_migration;
>  }
> -#else
> +#else /* CONFIG_SCHED_DEBUG */
>  static inline unsigned int get_sysctl_timer_migration(void)
>  {
>  	return 1;
>  }
> -#endif
> +#endif /* CONFIG_SCHED_DEBUG */
>  extern unsigned int sysctl_sched_rt_period;
>  extern int sysctl_sched_rt_runtime;
>  
> Index: tip/kernel/sched/core.c
> ===================================================================
> --- tip.orig/kernel/sched/core.c
> +++ tip/kernel/sched/core.c
> @@ -1533,6 +1533,21 @@ static void __sched_fork(struct task_str
>  #ifdef CONFIG_PREEMPT_NOTIFIERS
>  	INIT_HLIST_HEAD(&p->preempt_notifiers);
>  #endif
> +
> +#ifdef CONFIG_SCHED_NUMA
> +	if (p->mm && atomic_read(&p->mm->mm_users) == 1) {
> +		p->mm->numa_next_scan = jiffies;
> +		p->mm->numa_scan_seq = 0;
> +	}
> +
> +	p->node = -1;
> +	p->node_stamp = 0ULL;
> +	p->numa_scan_seq = p->mm ? p->mm->numa_scan_seq : 0;
> +	p->numa_migrate_seq = p->mm ? p->mm->numa_scan_seq - 1 : 0;
> +	p->numa_faults = NULL;
> +	p->numa_scan_period = sysctl_sched_numa_scan_period_min;
> +	p->numa_work.next = &p->numa_work;
> +#endif /* CONFIG_SCHED_NUMA */
>  }
>  
>  /*
> @@ -1774,6 +1789,7 @@ static void finish_task_switch(struct rq
>  	if (mm)
>  		mmdrop(mm);
>  	if (unlikely(prev_state == TASK_DEAD)) {
> +		task_numa_free(prev);
>  		/*
>  		 * Remove function-return probe instances associated with this
>  		 * task and put them back on the free list.
> Index: tip/kernel/sched/fair.c
> ===================================================================
> --- tip.orig/kernel/sched/fair.c
> +++ tip/kernel/sched/fair.c
> @@ -27,6 +27,8 @@
>  #include <linux/profile.h>
>  #include <linux/interrupt.h>
>  #include <linux/random.h>
> +#include <linux/mempolicy.h>
> +#include <linux/task_work.h>
>  
>  #include <trace/events/sched.h>
>  
> @@ -775,6 +777,21 @@ update_stats_curr_start(struct cfs_rq *c
>  
>  /**************************************************
>   * Scheduling class numa methods.
> + *
> + * The purpose of the NUMA bits are to maintain compute (task) and data
> + * (memory) locality. We try and achieve this by making tasks stick to
> + * a particular node (their home node) but if fairness mandates they run
> + * elsewhere for long enough, we let the memory follow them.
> + *
> + * Tasks start out with their home-node unset (-1) this effectively means
> + * they act !NUMA until we've established the task is busy enough to bother
> + * with placement.
> + *
> + * We keep a home-node per task and use periodic fault scans to try and
> + * estalish a task<->page relation. This assumes the task<->page relation is a
> + * compute<->data relation, this is false for things like virt. and n:m
> + * threading solutions but its the best we can do given the information we
> + * have.
>   */
>  
>  #ifdef CONFIG_SMP
> @@ -805,6 +822,157 @@ static void account_numa_dequeue(struct
>  	} else
>  		rq->onnode_running--;
>  }
> +
> +/*
> + * numa task sample period in ms: 5s
> + */
> +unsigned int sysctl_sched_numa_scan_period_min = 5000;
> +unsigned int sysctl_sched_numa_scan_period_max = 5000*16;
> +
> +/*
> + * Wait for the 2-sample stuff to settle before migrating again
> + */
> +unsigned int sysctl_sched_numa_settle_count = 2;
> +
> +static void task_numa_placement(struct task_struct *p)
> +{
> +	unsigned long faults, max_faults = 0;
> +	int node, max_node = -1;
> +	int seq = ACCESS_ONCE(p->mm->numa_scan_seq);
> +
> +	if (p->numa_scan_seq == seq)
> +		return;
> +
> +	p->numa_scan_seq = seq;
> +
> +	for (node = 0; node < nr_node_ids; node++) {
> +		faults = p->numa_faults[node];
> +
> +		if (faults > max_faults) {
> +			max_faults = faults;
> +			max_node = node;
> +		}
> +
> +		p->numa_faults[node] /= 2;
> +	}

No comments explaining the logic behind the decaying average. It can be
inferred if someone reads Documentation/scheduler/numa-problem.txt and
point 3c carefully enough. At the very least point them at it.


> +
> +	if (max_node == -1)
> +		return;
> +
> +	if (p->node != max_node) {
> +		p->numa_scan_period = sysctl_sched_numa_scan_period_min;
> +		if (sched_feat(NUMA_SETTLE) &&
> +		    (seq - p->numa_migrate_seq) <= (int)sysctl_sched_numa_settle_count)
> +			return;
> +		p->numa_migrate_seq = seq;
> +		sched_setnode(p, max_node);

Ok, so at a guess even if we do ping-pong it will only take effect every
10 seconds which could be far worse.

> +	} else {
> +		p->numa_scan_period = min(sysctl_sched_numa_scan_period_max,
> +				p->numa_scan_period * 2);
> +	}
> +}
> +
> +/*
> + * Got a PROT_NONE fault for a page on @node.
> + */
> +void task_numa_fault(int node, int pages)
> +{
> +	struct task_struct *p = current;
> +
> +	if (unlikely(!p->numa_faults)) {
> +		int size = sizeof(unsigned long) * nr_node_ids;
> +
> +		p->numa_faults = kzalloc(size, GFP_KERNEL);
> +		if (!p->numa_faults)
> +			return;
> +	}
> +

On a maximally configured machine this will be an order-4 allocation and
you need at least 512 nodes before it's an order-1 allocation. As unlikely
as it is, should this be GFP_NOWARN?

> +	task_numa_placement(p);
> +
> +	p->numa_faults[node] += pages;
> +}
> +
> +/*
> + * The expensive part of numa migration is done from task_work context.
> + * Triggered from task_tick_numa().
> + */
> +void task_numa_work(struct callback_head *work)
> +{
> +	unsigned long migrate, next_scan, now = jiffies;
> +	struct task_struct *p = current;
> +	struct mm_struct *mm = p->mm;
> +
> +	WARN_ON_ONCE(p != container_of(work, struct task_struct, numa_work));
> +
> +	work->next = work; /* protect against double add */
> +	/*
> +	 * Who cares about NUMA placement when they're dying.
> +	 *
> +	 * NOTE: make sure not to dereference p->mm before this check,
> +	 * exit_task_work() happens _after_ exit_mm() so we could be called
> +	 * without p->mm even though we still had it when we enqueued this
> +	 * work.
> +	 */
> +	if (p->flags & PF_EXITING)
> +		return;
> +
> +	/*
> +	 * Enforce maximal scan/migration frequency..
> +	 */
> +	migrate = mm->numa_next_scan;
> +	if (time_before(now, migrate))
> +		return;
> +
> +	next_scan = now + 2*msecs_to_jiffies(sysctl_sched_numa_scan_period_min);
> +	if (cmpxchg(&mm->numa_next_scan, migrate, next_scan) != migrate)
> +		return;
> +
> +	ACCESS_ONCE(mm->numa_scan_seq)++;
> +	{
> +		struct vm_area_struct *vma;
> +
> +		down_write(&mm->mmap_sem);
> +		for (vma = mm->mmap; vma; vma = vma->vm_next) {
> +			if (!vma_migratable(vma))
> +				continue;
> +			change_protection(vma, vma->vm_start, vma->vm_end, vma_prot_none(vma), 0);
> +		}
> +		up_write(&mm->mmap_sem);
> +	}
> +}

Ok, I like the idea of the scanning cost being incurred by the process.
I was going to complain though that for very large processes that the length
time it takes to complete this scan could be considerable.  However,
a quick glance forward indicates that you cope with this problem later by
limiting how much is scanned each time.

> +
> +/*
> + * Drive the periodic memory faults..
> + */
> +void task_tick_numa(struct rq *rq, struct task_struct *curr)
> +{
> +	struct callback_head *work = &curr->numa_work;
> +	u64 period, now;
> +
> +	/*
> +	 * We don't care about NUMA placement if we don't have memory.
> +	 */
> +	if (!curr->mm || (curr->flags & PF_EXITING) || work->next != work)
> +		return;
> +
> +	/*
> +	 * Using runtime rather than walltime has the dual advantage that
> +	 * we (mostly) drive the selection from busy threads and that the
> +	 * task needs to have done some actual work before we bother with
> +	 * NUMA placement.
> +	 */

Makes sense.

> +	now = curr->se.sum_exec_runtime;
> +	period = (u64)curr->numa_scan_period * NSEC_PER_MSEC;
> +
> +	if (now - curr->node_stamp > period) {
> +		curr->node_stamp = now;
> +
> +		if (!time_before(jiffies, curr->mm->numa_next_scan)) {
> +			init_task_work(work, task_numa_work); /* TODO: move this into sched_fork() */
> +			task_work_add(curr, work, true);
> +		}
> +	}
> +}
>  #else
>  #ifdef CONFIG_SMP
>  static struct list_head *account_numa_enqueue(struct rq *rq, struct task_struct *p)
> @@ -816,6 +984,10 @@ static struct list_head *account_numa_en
>  static void account_numa_dequeue(struct rq *rq, struct task_struct *p)
>  {
>  }
> +
> +static void task_tick_numa(struct rq *rq, struct task_struct *curr)
> +{
> +}
>  #endif /* CONFIG_SCHED_NUMA */
>  
>  /**************************************************
> @@ -5265,6 +5437,9 @@ static void task_tick_fair(struct rq *rq
>  		cfs_rq = cfs_rq_of(se);
>  		entity_tick(cfs_rq, se, queued);
>  	}
> +
> +	if (sched_feat_numa(NUMA))
> +		task_tick_numa(rq, curr);
>  }
>  
>  /*
> Index: tip/kernel/sched/features.h
> ===================================================================
> --- tip.orig/kernel/sched/features.h
> +++ tip/kernel/sched/features.h
> @@ -69,5 +69,6 @@ SCHED_FEAT(NUMA_TTWU_BIAS, false)
>  SCHED_FEAT(NUMA_TTWU_TO,   false)
>  SCHED_FEAT(NUMA_PULL,      true)
>  SCHED_FEAT(NUMA_PULL_BIAS, true)
> +SCHED_FEAT(NUMA_SETTLE,    true)
>  #endif
>  
> Index: tip/kernel/sched/sched.h
> ===================================================================
> --- tip.orig/kernel/sched/sched.h
> +++ tip/kernel/sched/sched.h
> @@ -3,6 +3,7 @@
>  #include <linux/mutex.h>
>  #include <linux/spinlock.h>
>  #include <linux/stop_machine.h>
> +#include <linux/slab.h>
>  
>  #include "cpupri.h"
>  
> @@ -476,15 +477,6 @@ struct rq {
>  #endif
>  };
>  
> -static inline struct list_head *offnode_tasks(struct rq *rq)
> -{
> -#ifdef CONFIG_SCHED_NUMA
> -	return &rq->offnode_tasks;
> -#else
> -	return NULL;
> -#endif
> -}
> -
>  static inline int cpu_of(struct rq *rq)
>  {
>  #ifdef CONFIG_SMP
> @@ -502,6 +494,27 @@ DECLARE_PER_CPU(struct rq, runqueues);
>  #define cpu_curr(cpu)		(cpu_rq(cpu)->curr)
>  #define raw_rq()		(&__raw_get_cpu_var(runqueues))
>  
> +#ifdef CONFIG_SCHED_NUMA
> +static inline struct list_head *offnode_tasks(struct rq *rq)
> +{
> +	return &rq->offnode_tasks;
> +}
> +
> +static inline void task_numa_free(struct task_struct *p)
> +{
> +	kfree(p->numa_faults);
> +}
> +#else /* CONFIG_SCHED_NUMA */
> +static inline struct list_head *offnode_tasks(struct rq *rq)
> +{
> +	return NULL;
> +}
> +
> +static inline void task_numa_free(struct task_struct *p)
> +{
> +}
> +#endif /* CONFIG_SCHED_NUMA */
> +
>  #ifdef CONFIG_SMP
>  
>  #define rcu_dereference_check_sched_domain(p) \
> Index: tip/kernel/sysctl.c
> ===================================================================
> --- tip.orig/kernel/sysctl.c
> +++ tip/kernel/sysctl.c
> @@ -256,9 +256,11 @@ static int min_sched_granularity_ns = 10
>  static int max_sched_granularity_ns = NSEC_PER_SEC;	/* 1 second */
>  static int min_wakeup_granularity_ns;			/* 0 usecs */
>  static int max_wakeup_granularity_ns = NSEC_PER_SEC;	/* 1 second */
> +#ifdef CONFIG_SMP
>  static int min_sched_tunable_scaling = SCHED_TUNABLESCALING_NONE;
>  static int max_sched_tunable_scaling = SCHED_TUNABLESCALING_END-1;
> -#endif
> +#endif /* CONFIG_SMP */
> +#endif /* CONFIG_SCHED_DEBUG */
>  
>  #ifdef CONFIG_COMPACTION
>  static int min_extfrag_threshold;
> @@ -301,6 +303,7 @@ static struct ctl_table kern_table[] = {
>  		.extra1		= &min_wakeup_granularity_ns,
>  		.extra2		= &max_wakeup_granularity_ns,
>  	},
> +#ifdef CONFIG_SMP
>  	{
>  		.procname	= "sched_tunable_scaling",
>  		.data		= &sysctl_sched_tunable_scaling,
> @@ -347,7 +350,31 @@ static struct ctl_table kern_table[] = {
>  		.extra1		= &zero,
>  		.extra2		= &one,
>  	},
> -#endif
> +#endif /* CONFIG_SMP */
> +#ifdef CONFIG_SCHED_NUMA
> +	{
> +		.procname	= "sched_numa_scan_period_min_ms",
> +		.data		= &sysctl_sched_numa_scan_period_min,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec,
> +	},
> +	{
> +		.procname	= "sched_numa_scan_period_max_ms",
> +		.data		= &sysctl_sched_numa_scan_period_max,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec,
> +	},
> +	{
> +		.procname	= "sched_numa_settle_count",
> +		.data		= &sysctl_sched_numa_settle_count,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec,
> +	},
> +#endif /* CONFIG_SCHED_NUMA */
> +#endif /* CONFIG_SCHED_DEBUG */
>  	{
>  		.procname	= "sched_rt_period_us",
>  		.data		= &sysctl_sched_rt_period,
> Index: tip/mm/huge_memory.c
> ===================================================================
> --- tip.orig/mm/huge_memory.c
> +++ tip/mm/huge_memory.c
> @@ -774,9 +774,10 @@ fixup:
>  
>  unlock:
>  	spin_unlock(&mm->page_table_lock);
> -	if (page)
> +	if (page) {
> +		task_numa_fault(page_to_nid(page), HPAGE_PMD_NR);
>  		put_page(page);
> -
> +	}
>  	return;
>  
>  migrate:
> @@ -845,6 +846,8 @@ migrate:
>  
>  	put_page(page);			/* Drop the rmap reference */
>  
> +	task_numa_fault(node, HPAGE_PMD_NR);
> +
>  	if (lru)
>  		put_page(page);		/* drop the LRU isolation reference */
>  
> Index: tip/mm/memory.c
> ===================================================================
> --- tip.orig/mm/memory.c
> +++ tip/mm/memory.c
> @@ -3512,8 +3512,10 @@ out_pte_upgrade_unlock:
>  out_unlock:
>  	pte_unmap_unlock(ptep, ptl);
>  out:
> -	if (page)
> +	if (page) {
> +		task_numa_fault(page_nid, 1);
>  		put_page(page);
> +	}
>  
>  	return 0;
>  
> 
> 

-- 
Mel Gorman
SUSE Labs
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