Date:	Sat, 26 Jul 2008 12:40:53 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	mingo@...e.hu
CC:	linux-kernel@...r.kernel.org, pj@....com, menage@...gle.com,
	a.p.zijlstra@...llo.nl, vegard.nossum@...il.com
Subject: Re: [PATCH] cpuset: Rework sched domains and CPU hotplug handling



Max Krasnyansky wrote:
> This is an updated version of my previous cpuset patch:
>   "Make rebuild_sched_domains() usable from any context (take 2)"


Folks,

Any comments on this patch ? We need this to complete sched domain handling
fixes/improvements that we started with the cpu_active_map, and to avoid
circular locking issues in the cpu hotplug -> rebuild_sched_domains path.

Max



> rebuild_sched_domains() is the only way to rebuild sched domains
> correctly based on the current cpuset settings. What this means
> is that we need to be able to call it from different contexts,
> like cpu hotplug for example.
> Also latest scheduler code in -tip now calls rebuild_sched_domains()
> directly from functions like arch_reinit_sched_domains().
> 
> In order to support that properly we need to rework cpuset locking
> rules to avoid circular depencies, which is what this patch does.
> New lock nesting rules are explained in the comments.
> We can now safely call rebuild_sched_domains() from virtually any
> context. The only requirement is that it needs to be called under
> get_online_cpus(). This allows cpu hotplug handlers and the scheduler
> to call rebuild_sched_domains() directly.
> 
> The rest of the cpuset code now offloads sched domains rebuilds to
> the single threaded workqueue. As suggested by both Paul J. and Paul M.
> 
> This version of the patch addresses comments from the previous review.
> I fixed all miss-formated comments and trailing spaces.
> __rebuild_sched_domains() has been renamed to async_rebuild_sched_domains().
> 
> I also factored out the code that builds domain masks and split up CPU and
> memory hotplug handling. This was needed to simplify locking, to avoid unsafe
> access to the cpu_online_map from mem hotplug handler, and in general to make
> things cleaner.
> 
> The patch passes moderate testing (building kernel with -j 16, creating &
> removing domains and bring cpus off/online at the same time) on the
> quad-core2 based machine.
> It passes lockdep checks, even with preemptable RCU enabled.
> 
> Signed-off-by: Max Krasnyansky <maxk@...lcomm.com>
> Cc: mingo@...e.hu
> Cc: pj@....com
> Cc: menage@...gle.com
> Cc: a.p.zijlstra@...llo.nl
> Cc: vegard.nossum@...il.com
> ---
>  kernel/cpuset.c |  328 ++++++++++++++++++++++++++++++++-----------------------
>  1 files changed, 191 insertions(+), 137 deletions(-)
> 
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index 3c3ef02..8121770 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -14,6 +14,8 @@
>   *  2003-10-22 Updates by Stephen Hemminger.
>   *  2004 May-July Rework by Paul Jackson.
>   *  2006 Rework by Paul Menage to use generic cgroups
> + *  2008 Rework of the scheduler domains and CPU hotplug handling
> + *       by Max Krasnyansky
>   *
>   *  This file is subject to the terms and conditions of the GNU General Public
>   *  License.  See the file COPYING in the main directory of the Linux
> @@ -59,6 +61,11 @@
>  #include <linux/cgroup.h>
>  
>  /*
> + * Workqueue for cpuset related tasks.
> + */
> +static struct workqueue_struct *cpuset_wq;
> +
> +/*
>   * Tracks how many cpusets are currently defined in system.
>   * When there is only one cpuset (the root cpuset) we can
>   * short circuit some hooks.
> @@ -241,9 +248,11 @@ static struct cpuset top_cpuset = {
>  
>  static DEFINE_MUTEX(callback_mutex);
>  
> -/* This is ugly, but preserves the userspace API for existing cpuset
> +/*
> + * This is ugly, but preserves the userspace API for existing cpuset
>   * users. If someone tries to mount the "cpuset" filesystem, we
> - * silently switch it to mount "cgroup" instead */
> + * silently switch it to mount "cgroup" instead
> + */
>  static int cpuset_get_sb(struct file_system_type *fs_type,
>  			 int flags, const char *unused_dev_name,
>  			 void *data, struct vfsmount *mnt)
> @@ -478,10 +487,9 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
>  }
>  
>  /*
> - * Helper routine for rebuild_sched_domains().
> + * Helper routine for generate_sched_domains().
>   * Do cpusets a, b have overlapping cpus_allowed masks?
>   */
> -
>  static int cpusets_overlap(struct cpuset *a, struct cpuset *b)
>  {
>  	return cpus_intersects(a->cpus_allowed, b->cpus_allowed);
> @@ -498,21 +506,41 @@ update_domain_attr(struct sched_domain_attr *dattr, struct cpuset *c)
>  }
>  
>  /*
> - * rebuild_sched_domains()
> - *
> - * If the flag 'sched_load_balance' of any cpuset with non-empty
> - * 'cpus' changes, or if the 'cpus' allowed changes in any cpuset
> - * which has that flag enabled, or if any cpuset with a non-empty
> - * 'cpus' is removed, then call this routine to rebuild the
> - * scheduler's dynamic sched domains.
> + * Helper routine for generate_sched_domains().
> + * Generates single, full, sched domain that includes all online
> + * CPUs in the system.
> + */
> +static int generate_single_sched_domain(cpumask_t **domains,
> +				struct sched_domain_attr **attributes)
> +{
> +	struct sched_domain_attr *dattr;
> +	cpumask_t *doms;
> +
> +	doms = kmalloc(sizeof(cpumask_t), GFP_KERNEL);
> +	if (!doms)
> +		return -ENOMEM;
> +	dattr = kmalloc(sizeof(struct sched_domain_attr), GFP_KERNEL);
> +	if (dattr) {
> +		*dattr = SD_ATTR_INIT;
> +		update_domain_attr(dattr, &top_cpuset);
> +	}
> +	*doms = top_cpuset.cpus_allowed;
> +
> +	*domains    = doms;
> +	*attributes = dattr;
> +	return 1;
> +}
> +
> +/*
> + * generate_sched_domains()
>   *
> - * This routine builds a partial partition of the systems CPUs
> - * (the set of non-overlappping cpumask_t's in the array 'part'
> - * below), and passes that partial partition to the kernel/sched.c
> - * partition_sched_domains() routine, which will rebuild the
> - * schedulers load balancing domains (sched domains) as specified
> - * by that partial partition.  A 'partial partition' is a set of
> - * non-overlapping subsets whose union is a subset of that set.
> + * This function builds a partial partition of the systems CPUs
> + * A 'partial partition' is a set of non-overlapping subsets whose
> + * union is a subset of that set.
> + * The output of this function needs to be passed to kernel/sched.c
> + * partition_sched_domains() routine, which will rebuild the scheduler's
> + * load balancing domains (sched domains) as specified by that partial
> + * partition.
>   *
>   * See "What is sched_load_balance" in Documentation/cpusets.txt
>   * for a background explanation of this.
> @@ -522,13 +550,7 @@ update_domain_attr(struct sched_domain_attr *dattr, struct cpuset *c)
>   * domains when operating in the severe memory shortage situations
>   * that could cause allocation failures below.
>   *
> - * Call with cgroup_mutex held.  May take callback_mutex during
> - * call due to the kfifo_alloc() and kmalloc() calls.  May nest
> - * a call to the get_online_cpus()/put_online_cpus() pair.
> - * Must not be called holding callback_mutex, because we must not
> - * call get_online_cpus() while holding callback_mutex.  Elsewhere
> - * the kernel nests callback_mutex inside get_online_cpus() calls.
> - * So the reverse nesting would risk an ABBA deadlock.
> + * Must be called with cgroup_lock held.
>   *
>   * The three key local variables below are:
>   *    q  - a kfifo queue of cpuset pointers, used to implement a
> @@ -563,8 +585,8 @@ update_domain_attr(struct sched_domain_attr *dattr, struct cpuset *c)
>   *	element of the partition (one sched domain) to be passed to
>   *	partition_sched_domains().
>   */
> -
> -void rebuild_sched_domains(void)
> +static int generate_sched_domains(cpumask_t **domains,
> +			struct sched_domain_attr **attributes)
>  {
>  	struct kfifo *q;	/* queue of cpusets to be scanned */
>  	struct cpuset *cp;	/* scans q */
> @@ -576,32 +598,23 @@ void rebuild_sched_domains(void)
>  	int ndoms;		/* number of sched domains in result */
>  	int nslot;		/* next empty doms[] cpumask_t slot */
>  
> -	q = NULL;
> -	csa = NULL;
> -	doms = NULL;
> +	doms  = NULL;
>  	dattr = NULL;
>  
>  	/* Special case for the 99% of systems with one, full, sched domain */
> -	if (is_sched_load_balance(&top_cpuset)) {
> -		ndoms = 1;
> -		doms = kmalloc(sizeof(cpumask_t), GFP_KERNEL);
> -		if (!doms)
> -			goto rebuild;
> -		dattr = kmalloc(sizeof(struct sched_domain_attr), GFP_KERNEL);
> -		if (dattr) {
> -			*dattr = SD_ATTR_INIT;
> -			update_domain_attr(dattr, &top_cpuset);
> -		}
> -		*doms = top_cpuset.cpus_allowed;
> -		goto rebuild;
> -	}
> +	if (is_sched_load_balance(&top_cpuset))
> +		return generate_single_sched_domain(domains, attributes);
>  
>  	q = kfifo_alloc(number_of_cpusets * sizeof(cp), GFP_KERNEL, NULL);
> -	if (IS_ERR(q))
> -		goto done;
> +	if (!q || IS_ERR(q))
> +		return 0;
> +
>  	csa = kmalloc(number_of_cpusets * sizeof(cp), GFP_KERNEL);
> -	if (!csa)
> -		goto done;
> +	if (!csa) {
> +		kfifo_free(q);
> +		return 0;
> +	}
> +
>  	csn = 0;
>  
>  	cp = &top_cpuset;
> @@ -644,61 +657,119 @@ restart:
>  		}
>  	}
>  
> -	/* Convert <csn, csa> to <ndoms, doms> */
> +	/*
> +	 * Now we know how many domains to create.
> +	 * Convert <csn, csa> to <ndoms, doms> and populate cpu masks.
> +	 */
>  	doms = kmalloc(ndoms * sizeof(cpumask_t), GFP_KERNEL);
> -	if (!doms)
> -		goto rebuild;
> +	if (!doms) {
> +		ndoms = 0;
> +		goto done;
> +	}
>  	dattr = kmalloc(ndoms * sizeof(struct sched_domain_attr), GFP_KERNEL);
>  
>  	for (nslot = 0, i = 0; i < csn; i++) {
>  		struct cpuset *a = csa[i];
>  		int apn = a->pn;
> +		cpumask_t *dp = doms + nslot;
>  
> -		if (apn >= 0) {
> -			cpumask_t *dp = doms + nslot;
> -
> -			if (nslot == ndoms) {
> -				static int warnings = 10;
> -				if (warnings) {
> -					printk(KERN_WARNING
> -					 "rebuild_sched_domains confused:"
> -					  " nslot %d, ndoms %d, csn %d, i %d,"
> -					  " apn %d\n",
> -					  nslot, ndoms, csn, i, apn);
> -					warnings--;
> -				}
> -				continue;
> +		/* Skip partitions we've already looked at */
> +		if (apn < 0)
> +			continue;
> +
> +		if (nslot == ndoms) {
> +			static int warnings = 10;
> +			if (warnings) {
> +				printk(KERN_WARNING
> +					"rebuild_sched_domains confused:"
> +					" nslot %d, ndoms %d, csn %d, i %d,"
> +					" apn %d\n",
> +						nslot, ndoms, csn, i, apn);
> +				warnings--;
>  			}
> +			continue;
> +		}
>  
> -			cpus_clear(*dp);
> -			if (dattr)
> -				*(dattr + nslot) = SD_ATTR_INIT;
> -			for (j = i; j < csn; j++) {
> -				struct cpuset *b = csa[j];
> +		cpus_clear(*dp);
> +		if (dattr)
> +			*(dattr + nslot) = SD_ATTR_INIT;
> +		for (j = i; j < csn; j++) {
> +			struct cpuset *b = csa[j];
>  
> -				if (apn == b->pn) {
> -					cpus_or(*dp, *dp, b->cpus_allowed);
> -					b->pn = -1;
> -					update_domain_attr(dattr, b);
> -				}
> +			if (apn == b->pn) {
> +				cpus_or(*dp, *dp, b->cpus_allowed);
> +				update_domain_attr(dattr, b);
> +
> +				/* Done with this partition */
> +				b->pn = -1;
>  			}
> -			nslot++;
>  		}
> +		nslot++;
>  	}
>  	BUG_ON(nslot != ndoms);
>  
> -rebuild:
> -	/* Have scheduler rebuild sched domains */
> +done:
> +	kfifo_free(q);
> +	kfree(csa);
> +
> +	*domains    = doms;
> +	*attributes = dattr;
> +	return ndoms;
> +}
> +
> +/*
> + * Rebuilds scheduler domains. See generate_sched_domains() description
> + * for details.
> + *
> + * Must be called under get_online_cpus(). This is an external interface
> + * and must not be used inside the cpuset code to avoid circular locking
> + * dependency. cpuset code should use async_rebuild_sched_domains() instead.
> + */
> +void rebuild_sched_domains(void)
> +{
> +	struct sched_domain_attr *attr;
> +	cpumask_t *doms;
> +	int ndoms;
> +
> +	/*
> +	 * We have to iterate cgroup hierarch which requires cgroup lock.
> +	 */
> +	cgroup_lock();
> +	ndoms = generate_sched_domains(&doms, &attr);
> +	cgroup_unlock();
> +
> +	/* Have scheduler rebuild the domains */
> +	partition_sched_domains(ndoms, doms, attr);
> +}
> +
> +/*
> + * Simply calls rebuild_sched_domains() with correct locking rules.
> + */
> +static void rebuild_domains_callback(struct work_struct *work)
> +{
>  	get_online_cpus();
> -	partition_sched_domains(ndoms, doms, dattr);
> +	rebuild_sched_domains();
>  	put_online_cpus();
> +}
> +static DECLARE_WORK(rebuild_domains_work, rebuild_domains_callback);
>  
> -done:
> -	if (q && !IS_ERR(q))
> -		kfifo_free(q);
> -	kfree(csa);
> -	/* Don't kfree(doms) -- partition_sched_domains() does that. */
> -	/* Don't kfree(dattr) -- partition_sched_domains() does that. */
> +/*
> + * Internal helper for rebuilding sched domains when something changes.
> + *
> + * If the flag 'sched_load_balance' of any cpuset with non-empty
> + * 'cpus' changes, or if the 'cpus' allowed changes in any cpuset
> + * which has that flag enabled, or if any cpuset with a non-empty
> + * 'cpus' is removed, then call this routine to rebuild the
> + * scheduler's dynamic sched domains.
> + *
> + * rebuild_sched_domains() must be called under get_online_cpus() and
> + * it needs to take cgroup_lock(). But most of the cpuset code is already
> + * holding cgroup_lock(). In order to avoid incorrect lock nesting we
> + * delegate the actual domain rebuilding to the workqueue.
> + */
> +static void async_rebuild_sched_domains(void)
> +{
> +	queue_work(cpuset_wq, &rebuild_domains_work);
>  }
>  
>  static inline int started_after_time(struct task_struct *t1,
> @@ -831,7 +902,7 @@ static int update_cpumask(struct cpuset *cs, char *buf)
>  	heap_free(&heap);
>  
>  	if (is_load_balanced)
> -		rebuild_sched_domains();
> +		async_rebuild_sched_domains();
>  	return 0;
>  }
>  
> @@ -1042,7 +1113,7 @@ static int update_relax_domain_level(struct cpuset *cs, s64 val)
>  
>  	if (val != cs->relax_domain_level) {
>  		cs->relax_domain_level = val;
> -		rebuild_sched_domains();
> +		async_rebuild_sched_domains();
>  	}
>  
>  	return 0;
> @@ -1083,7 +1154,7 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
>  	mutex_unlock(&callback_mutex);
>  
>  	if (cpus_nonempty && balance_flag_changed)
> -		rebuild_sched_domains();
> +		async_rebuild_sched_domains();
>  
>  	return 0;
>  }
> @@ -1479,6 +1550,9 @@ static u64 cpuset_read_u64(struct cgroup *cont, struct cftype *cft)
>  	default:
>  		BUG();
>  	}
> +
> +	/* Unreachable but makes gcc happy */
> +	return 0;
>  }
>  
>  static s64 cpuset_read_s64(struct cgroup *cont, struct cftype *cft)
> @@ -1491,6 +1565,9 @@ static s64 cpuset_read_s64(struct cgroup *cont, struct cftype *cft)
>  	default:
>  		BUG();
>  	}
> +
> +	/* Unrechable but makes gcc happy */
> +	return 0;
>  }
>  
>  
> @@ -1677,15 +1754,9 @@ static struct cgroup_subsys_state *cpuset_create(
>  }
>  
>  /*
> - * Locking note on the strange update_flag() call below:
> - *
>   * If the cpuset being removed has its flag 'sched_load_balance'
>   * enabled, then simulate turning sched_load_balance off, which
> - * will call rebuild_sched_domains().  The get_online_cpus()
> - * call in rebuild_sched_domains() must not be made while holding
> - * callback_mutex.  Elsewhere the kernel nests callback_mutex inside
> - * get_online_cpus() calls.  So the reverse nesting would risk an
> - * ABBA deadlock.
> + * will call rebuild_sched_domains().
>   */
>  
>  static void cpuset_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
> @@ -1796,7 +1867,7 @@ static void move_member_tasks_to_cpuset(struct cpuset *from, struct cpuset *to)
>  }
>  
>  /*
> - * If common_cpu_mem_hotplug_unplug(), below, unplugs any CPUs
> + * If CPU and/or memory hotplug handlers, below, unplug any CPUs
>   * or memory nodes, we need to walk over the cpuset hierarchy,
>   * removing that CPU or node from all cpusets.  If this removes the
>   * last CPU or node from a cpuset, then move the tasks in the empty
> @@ -1884,35 +1955,6 @@ static void scan_for_empty_cpusets(const struct cpuset *root)
>  }
>  
>  /*
> - * The cpus_allowed and mems_allowed nodemasks in the top_cpuset track
> - * cpu_online_map and node_states[N_HIGH_MEMORY].  Force the top cpuset to
> - * track what's online after any CPU or memory node hotplug or unplug event.
> - *
> - * Since there are two callers of this routine, one for CPU hotplug
> - * events and one for memory node hotplug events, we could have coded
> - * two separate routines here.  We code it as a single common routine
> - * in order to minimize text size.
> - */
> -
> -static void common_cpu_mem_hotplug_unplug(int rebuild_sd)
> -{
> -	cgroup_lock();
> -
> -	top_cpuset.cpus_allowed = cpu_online_map;
> -	top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
> -	scan_for_empty_cpusets(&top_cpuset);
> -
> -	/*
> -	 * Scheduler destroys domains on hotplug events.
> -	 * Rebuild them based on the current settings.
> -	 */
> -	if (rebuild_sd)
> -		rebuild_sched_domains();
> -
> -	cgroup_unlock();
> -}
> -
> -/*
>   * The top_cpuset tracks what CPUs and Memory Nodes are online,
>   * period.  This is necessary in order to make cpusets transparent
>   * (of no affect) on systems that are actively using CPU hotplug
> @@ -1921,39 +1963,48 @@ static void common_cpu_mem_hotplug_unplug(int rebuild_sd)
>   * This routine ensures that top_cpuset.cpus_allowed tracks
>   * cpu_online_map on each CPU hotplug (cpuhp) event.
>   */
> -
> -static int cpuset_handle_cpuhp(struct notifier_block *unused_nb,
> +static int cpuset_track_online_cpus(struct notifier_block *unused_nb,
>  				unsigned long phase, void *unused_cpu)
>  {
> +	struct sched_domain_attr *attr;
> +	cpumask_t *doms;
> +	int ndoms;
> +
>  	switch (phase) {
> -	case CPU_UP_CANCELED:
> -	case CPU_UP_CANCELED_FROZEN:
> -	case CPU_DOWN_FAILED:
> -	case CPU_DOWN_FAILED_FROZEN:
>  	case CPU_ONLINE:
>  	case CPU_ONLINE_FROZEN:
>  	case CPU_DEAD:
>  	case CPU_DEAD_FROZEN:
> -		common_cpu_mem_hotplug_unplug(1);
>  		break;
> +
>  	default:
>  		return NOTIFY_DONE;
>  	}
>  
> +	cgroup_lock();
> +	top_cpuset.cpus_allowed = cpu_online_map;
> +	scan_for_empty_cpusets(&top_cpuset);
> +	ndoms = generate_sched_domains(&doms, &attr);
> +	cgroup_unlock();
> +
> +	/* Have scheduler rebuild the domains */
> +	partition_sched_domains(ndoms, doms, attr);
> +
>  	return NOTIFY_OK;
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  /*
>   * Keep top_cpuset.mems_allowed tracking node_states[N_HIGH_MEMORY].
> - * Call this routine anytime after you change
> - * node_states[N_HIGH_MEMORY].
> - * See also the previous routine cpuset_handle_cpuhp().
> + * Call this routine anytime after node_states[N_HIGH_MEMORY] changes.
> + * See also the previous routine cpuset_track_online_cpus().
>   */
> -
>  void cpuset_track_online_nodes(void)
>  {
> -	common_cpu_mem_hotplug_unplug(0);
> +	cgroup_lock();
> +	top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
> +	scan_for_empty_cpusets(&top_cpuset);
> +	cgroup_unlock();
>  }
>  #endif
>  
> @@ -1968,7 +2019,10 @@ void __init cpuset_init_smp(void)
>  	top_cpuset.cpus_allowed = cpu_online_map;
>  	top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
>  
> -	hotcpu_notifier(cpuset_handle_cpuhp, 0);
> +	hotcpu_notifier(cpuset_track_online_cpus, 0);
> +
> +	cpuset_wq = create_singlethread_workqueue("cpuset");
> +	BUG_ON(!cpuset_wq);
>  }
>  
>  /**
--
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