Date:	Thu, 26 Apr 2012 21:41:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 6/6] rcu: Reduce cache-miss
 initialization latencies for large systems

On Thu, 2012-04-26 at 09:15 -0700, Paul E. McKenney wrote:
> On Thu, Apr 26, 2012 at 05:28:57PM +0200, Peter Zijlstra wrote:
> > On Thu, 2012-04-26 at 07:12 -0700, Paul E. McKenney wrote:
> > > On Thu, Apr 26, 2012 at 02:51:47PM +0200, Peter Zijlstra wrote:
> > 
> > > > Wouldn't it be much better to match the rcu fanout tree to the physical
> > > > topology of the machine?
> > > 
> > > From what I am hearing, doing so requires me to morph the rcu_node tree
> > > at run time.  I might eventually become courageous/inspired/senile
> > > enough to try this, but not yet.  ;-)
> > 
> > Yes, boot time with possibly some hotplug hooks.
> 
> Has anyone actually measured any slowdown due to the rcu_node structure
> not matching the topology?  (But see also below.)

Nope, I'm just whinging ;-)

> > > Actually, some of this topology shifting seems to me like a firmware
> > > bug.  Why not arrange the Linux-visible numbering in a way to promote
> > > locality for code sequencing through the CPUs?
> > 
> > I'm not sure.. but it seems well established on x86 to first enumerate
> > the cores (thread 0) and then the sibling threads (thread 1) -- one
> > 'advantage' is that if you boot with max_cpus=$half you get all cores
> > instead of half the cores.
> > 
> > OTOH it does make linear iteration of the cpus 'funny' :-)
> 
> Like I said, firmware bug.  Seems like the fix should be there as well.
> Perhaps there needs to be two CPU numberings, one for people wanting
> whole cores and another for people who want cache locality.  Yes, this
> could be confusing, but keep in mind that you are asking every kernel
> subsystem to keep its own version of the cache-locality numbering,
> and that will be even more confusing.

I really don't see why it would matter, as far I care they're completely
randomized on boot, its all done using cpu-bitmasks anyway.

Suppose the linear thing would have threads/cores/cache continuity like
you want, that still leaves the node interconnects, and we all know
people love to be creative with those, no way you're going to fold that
into a linear scheme :-)

Anyway, as it currently stands I can offer you: cpus_share_cache(),
which will return true/false depending on if the two cpus do indeed
share a cache.

On top of that there's node_distance(), which will (hopefully) reflect
the interconnect topology between nodes.

Using these you can construct enough of the topology layout to be
useful. NOTE: node topologies don't need to be symmetric!

> > Also, a fanout of 16 is nice when your machine doesn't have HT and has a
> > 2^n core count, but some popular machines these days have 6/10 cores per
> > socket, resulting in your fanout splitting caches.
> 
> That is easy.  Such systems can set CONFIG_RCU_FANOUT to 6, 12, 10,
> or 20, depending on preference.  With a patch intended for 3.6, they
> could set the smallest reasonable value at build time and adjust to
> the hardware using the boot parameter.
> 
> http://www.gossamer-threads.com/lists/linux/kernel/1524864
> 
> I expect to make other similar changes over time, but will be proceeding
> cautiously.

I can very easily give you the size (nr cpus in) a node, still as long
as you iterate the cpu space linearly that's not going to be much help.

I can also offer you access to the scheduler topology if you want.. I've
got the below patch pending which should (hopefully) improve the
scheduler's node topology -- it implements that detection based on
node_distance().

I've tested it using some fake-numa hacks to feed it the distance table
of an AMD quad-socket Magny-Cours:

 "numa=fake=8:10,16,16,22,16,22,16,22,
              16,10,22,16,22,16,22,16,
              16,22,10,16,16,22,16,22,
              22,16,16,10,22,16,22,16,
              16,22,16,22,10,16,16,22,
              22,16,22,16,16,10,22,16,
              16,22,16,22,16,22,10,16,
              22,16,22,16,22,16,16,10"



---
Subject: sched: Rewrite the CONFIG_NUMA sched domain support.
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date: Tue Apr 17 15:49:36 CEST 2012

The current code groups up to 16 nodes in a level and then puts an
ALLNODES domain spanning the entire tree on top of that. This doesn't
reflect the numa topology and esp for the smaller not-fully-connected
machines out there today this might make a difference.

Therefore, build a proper numa topology based on node_distance().

TODO: figure out a way to set SD_flags based on distance such that
      we disable various expensive load-balancing features at some
      point and increase the balance interval prop. to the distance.

Cc: Anton Blanchard <anton@...ba.org>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Chris Metcalf <cmetcalf@...era.com>
Cc: David Howells <dhowells@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Fenghua Yu <fenghua.yu@...el.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
Cc: linux-alpha@...r.kernel.org
Cc: linux-ia64@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-mips@...ux-mips.org
Cc: linuxppc-dev@...ts.ozlabs.org
Cc: linux-sh@...r.kernel.org
Cc: Matt Turner <mattst88@...il.com>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Paul Mundt <lethal@...ux-sh.org>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Cc: Richard Henderson <rth@...ddle.net>
Cc: sparclinux@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Tony Luck <tony.luck@...el.com>
Cc: x86@...nel.org
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/n/tip-fgj6245hxj61qe8vy7c6cmjj@git.kernel.org
---
 arch/powerpc/include/asm/topology.h |    6 
 arch/x86/include/asm/topology.h     |   38 -----
 include/linux/topology.h            |   36 -----
 kernel/sched/core.c                 |  253 ++++++++++++++++++++++--------------
 4 files changed, 158 insertions(+), 175 deletions(-)

Index: linux-2.6/include/linux/topology.h
===================================================================
--- linux-2.6.orig/include/linux/topology.h
+++ linux-2.6/include/linux/topology.h
@@ -176,48 +176,12 @@ int arch_update_cpu_topology(void);
 }
 #endif
 
-/* sched_domains SD_ALLNODES_INIT for NUMA machines */
-#define SD_ALLNODES_INIT (struct sched_domain) {			\
-	.min_interval		= 64,					\
-	.max_interval		= 64*num_online_cpus(),			\
-	.busy_factor		= 128,					\
-	.imbalance_pct		= 133,					\
-	.cache_nice_tries	= 1,					\
-	.busy_idx		= 3,					\
-	.idle_idx		= 3,					\
-	.flags			= 1*SD_LOAD_BALANCE			\
-				| 1*SD_BALANCE_NEWIDLE			\
-				| 0*SD_BALANCE_EXEC			\
-				| 0*SD_BALANCE_FORK			\
-				| 0*SD_BALANCE_WAKE			\
-				| 0*SD_WAKE_AFFINE			\
-				| 0*SD_SHARE_CPUPOWER			\
-				| 0*SD_POWERSAVINGS_BALANCE		\
-				| 0*SD_SHARE_PKG_RESOURCES		\
-				| 1*SD_SERIALIZE			\
-				| 0*SD_PREFER_SIBLING			\
-				,					\
-	.last_balance		= jiffies,				\
-	.balance_interval	= 64,					\
-}
-
-#ifndef SD_NODES_PER_DOMAIN
-#define SD_NODES_PER_DOMAIN 16
-#endif
-
 #ifdef CONFIG_SCHED_BOOK
 #ifndef SD_BOOK_INIT
 #error Please define an appropriate SD_BOOK_INIT in include/asm/topology.h!!!
 #endif
 #endif /* CONFIG_SCHED_BOOK */
 
-#ifdef CONFIG_NUMA
-#ifndef SD_NODE_INIT
-#error Please define an appropriate SD_NODE_INIT in include/asm/topology.h!!!
-#endif
-
-#endif /* CONFIG_NUMA */
-
 #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
 DECLARE_PER_CPU(int, numa_node);
 
Index: linux-2.6/kernel/sched/core.c
===================================================================
--- linux-2.6.orig/kernel/sched/core.c
+++ linux-2.6/kernel/sched/core.c
@@ -5560,7 +5560,8 @@ static int sched_domain_debug_one(struct
 			break;
 		}
 
-		if (cpumask_intersects(groupmask, sched_group_cpus(group))) {
+		if (!(sd->flags & SD_OVERLAP) &&
+		    cpumask_intersects(groupmask, sched_group_cpus(group))) {
 			printk(KERN_CONT "\n");
 			printk(KERN_ERR "ERROR: repeated CPUs\n");
 			break;
@@ -5898,92 +5899,6 @@ static int __init isolated_cpu_setup(cha
 
 __setup("isolcpus=", isolated_cpu_setup);
 
-#ifdef CONFIG_NUMA
-
-/**
- * find_next_best_node - find the next node to include in a sched_domain
- * @node: node whose sched_domain we're building
- * @used_nodes: nodes already in the sched_domain
- *
- * Find the next node to include in a given scheduling domain. Simply
- * finds the closest node not already in the @used_nodes map.
- *
- * Should use nodemask_t.
- */
-static int find_next_best_node(int node, nodemask_t *used_nodes)
-{
-	int i, n, val, min_val, best_node = -1;
-
-	min_val = INT_MAX;
-
-	for (i = 0; i < nr_node_ids; i++) {
-		/* Start at @node */
-		n = (node + i) % nr_node_ids;
-
-		if (!nr_cpus_node(n))
-			continue;
-
-		/* Skip already used nodes */
-		if (node_isset(n, *used_nodes))
-			continue;
-
-		/* Simple min distance search */
-		val = node_distance(node, n);
-
-		if (val < min_val) {
-			min_val = val;
-			best_node = n;
-		}
-	}
-
-	if (best_node != -1)
-		node_set(best_node, *used_nodes);
-	return best_node;
-}
-
-/**
- * sched_domain_node_span - get a cpumask for a node's sched_domain
- * @node: node whose cpumask we're constructing
- * @span: resulting cpumask
- *
- * Given a node, construct a good cpumask for its sched_domain to span. It
- * should be one that prevents unnecessary balancing, but also spreads tasks
- * out optimally.
- */
-static void sched_domain_node_span(int node, struct cpumask *span)
-{
-	nodemask_t used_nodes;
-	int i;
-
-	cpumask_clear(span);
-	nodes_clear(used_nodes);
-
-	cpumask_or(span, span, cpumask_of_node(node));
-	node_set(node, used_nodes);
-
-	for (i = 1; i < SD_NODES_PER_DOMAIN; i++) {
-		int next_node = find_next_best_node(node, &used_nodes);
-		if (next_node < 0)
-			break;
-		cpumask_or(span, span, cpumask_of_node(next_node));
-	}
-}
-
-static const struct cpumask *cpu_node_mask(int cpu)
-{
-	lockdep_assert_held(&sched_domains_mutex);
-
-	sched_domain_node_span(cpu_to_node(cpu), sched_domains_tmpmask);
-
-	return sched_domains_tmpmask;
-}
-
-static const struct cpumask *cpu_allnodes_mask(int cpu)
-{
-	return cpu_possible_mask;
-}
-#endif /* CONFIG_NUMA */
-
 static const struct cpumask *cpu_cpu_mask(int cpu)
 {
 	return cpumask_of_node(cpu_to_node(cpu));
@@ -6020,6 +5935,7 @@ struct sched_domain_topology_level {
 	sched_domain_init_f init;
 	sched_domain_mask_f mask;
 	int		    flags;
+	int		    numa_level;
 	struct sd_data      data;
 };
 
@@ -6211,10 +6127,6 @@ sd_init_##type(struct sched_domain_topol
 }
 
 SD_INIT_FUNC(CPU)
-#ifdef CONFIG_NUMA
- SD_INIT_FUNC(ALLNODES)
- SD_INIT_FUNC(NODE)
-#endif
 #ifdef CONFIG_SCHED_SMT
  SD_INIT_FUNC(SIBLING)
 #endif
@@ -6336,15 +6248,164 @@ static struct sched_domain_topology_leve
 	{ sd_init_BOOK, cpu_book_mask, },
 #endif
 	{ sd_init_CPU, cpu_cpu_mask, },
-#ifdef CONFIG_NUMA
-	{ sd_init_NODE, cpu_node_mask, SDTL_OVERLAP, },
-	{ sd_init_ALLNODES, cpu_allnodes_mask, },
-#endif
 	{ NULL, },
 };
 
 static struct sched_domain_topology_level *sched_domain_topology = default_topology;
 
+#ifdef CONFIG_NUMA
+
+static int sched_domains_numa_levels;
+static int sched_domains_numa_scale;
+static int *sched_domains_numa_distance;
+static struct cpumask ** __percpu sched_domains_numa_masks;
+static int sched_domains_curr_level;
+
+#define NUMA_SCALE(x, level)	\
+
+static inline unsigned long numa_scale(unsigned long x, int level)
+{
+	return x * sched_domains_numa_distance[level] / sched_domains_numa_scale;
+}
+
+static inline int sd_local_flags(int level)
+{
+	if (sched_domains_numa_distance[level] > REMOTE_DISTANCE)
+		return 0;
+
+	return SD_BALANCE_EXEC | SD_BALANCE_FORK | SD_WAKE_AFFINE;
+}
+
+static struct sched_domain *
+sd_init_NUMA(struct sched_domain_topology_level *tl, int cpu)
+{
+	struct sched_domain *sd = *per_cpu_ptr(tl->data.sd, cpu);
+	int level = tl->numa_level;
+	int sd_weight = cpumask_weight(per_cpu_ptr(
+				sched_domains_numa_masks[level],
+				cpu));
+
+	*sd = (struct sched_domain){
+		.min_interval		= sd_weight,
+		.max_interval		= 2*sd_weight,
+		.busy_factor		= 32,
+		.imbalance_pct		= 100 + numa_scale(25, level),
+		.cache_nice_tries	= 2,
+		.busy_idx		= 3,
+		.idle_idx		= 2,
+		.newidle_idx		= 0,
+		.wake_idx		= 0,
+		.forkexec_idx		= 0,
+
+		.flags			= 1*SD_LOAD_BALANCE
+					| 1*SD_BALANCE_NEWIDLE
+					| 0*SD_BALANCE_EXEC
+					| 0*SD_BALANCE_FORK
+					| 0*SD_BALANCE_WAKE
+					| 0*SD_WAKE_AFFINE
+					| 0*SD_PREFER_LOCAL
+					| 0*SD_SHARE_CPUPOWER
+					| 0*SD_POWERSAVINGS_BALANCE
+					| 0*SD_SHARE_PKG_RESOURCES
+					| 1*SD_SERIALIZE
+					| 0*SD_PREFER_SIBLING
+					| sd_local_flags(level)
+					,
+		.last_balance		= jiffies,
+		.balance_interval	= sd_weight,
+	};
+	SD_INIT_NAME(sd, NUMA);
+	sd->private = &tl->data;
+
+	/*
+	 * Ugly hack to pass state to sd_numa_mask()...
+	 */
+	sched_domains_curr_level = tl->numa_level;
+
+	return sd;
+}
+
+static const struct cpumask *sd_numa_mask(int cpu)
+{
+	return per_cpu_ptr(sched_domains_numa_masks[sched_domains_curr_level], cpu);
+}
+
+static void sched_init_numa(void)
+{
+	int next_distance, curr_distance = node_distance(0, 0);
+	struct sched_domain_topology_level *tl;
+	int level = 0;
+	int i, j, k;
+
+	sched_domains_numa_scale = curr_distance;
+	sched_domains_numa_distance = kzalloc(sizeof(int) * nr_node_ids, GFP_KERNEL);
+	if (!sched_domains_numa_distance)
+		return;
+
+	next_distance = curr_distance;
+	for (i = 0; i < nr_node_ids; i++) {
+		for (j = 0; j < nr_node_ids; j++) {
+			int distance = node_distance(0, j);
+			if (distance > curr_distance &&
+					(distance < next_distance ||
+					 next_distance == curr_distance))
+				next_distance = distance;
+		}
+		if (next_distance != curr_distance) {
+			sched_domains_numa_distance[level++] = next_distance;
+			sched_domains_numa_levels = level;
+			curr_distance = next_distance;
+		} else break;
+	}
+
+	sched_domains_numa_masks = kzalloc(sizeof(void *) * level, GFP_KERNEL);
+	if (!sched_domains_numa_masks)
+		return;
+
+	for (i = 0; i < level; i++) {
+		sched_domains_numa_masks[i] = alloc_percpu(cpumask_t);
+		if (!sched_domains_numa_masks[i])
+			return;
+
+		for_each_possible_cpu(j) {
+			struct cpumask *mask =
+				per_cpu_ptr(sched_domains_numa_masks[i], j);
+
+			for (k = 0; k < nr_node_ids; k++) {
+				if (node_distance(cpu_to_node(j), k) >
+						sched_domains_numa_distance[i])
+					continue;
+
+				cpumask_or(mask, mask, cpumask_of_node(k));
+			}
+		}
+	}
+
+	tl = kzalloc((ARRAY_SIZE(default_topology) + level) *
+			sizeof(struct sched_domain_topology_level), GFP_KERNEL);
+	if (!tl)
+		return;
+
+	for (i = 0; default_topology[i].init; i++)
+		tl[i] = default_topology[i];
+
+	for (j = 0; j < level; i++, j++) {
+		tl[i] = (struct sched_domain_topology_level){
+			.init = sd_init_NUMA,
+			.mask = sd_numa_mask,
+			.flags = SDTL_OVERLAP,
+			.numa_level = j,
+		};
+	}
+
+	sched_domain_topology = tl;
+}
+#else
+static inline void sched_init_numa(void)
+{
+}
+#endif /* CONFIG_NUMA */
+
 static int __sdt_alloc(const struct cpumask *cpu_map)
 {
 	struct sched_domain_topology_level *tl;
@@ -6828,6 +6889,8 @@ void __init sched_init_smp(void)
 	alloc_cpumask_var(&non_isolated_cpus, GFP_KERNEL);
 	alloc_cpumask_var(&fallback_doms, GFP_KERNEL);
 
+	sched_init_numa();
+
 	get_online_cpus();
 	mutex_lock(&sched_domains_mutex);
 	init_sched_domains(cpu_active_mask);
Index: linux-2.6/arch/powerpc/include/asm/topology.h
===================================================================
--- linux-2.6.orig/arch/powerpc/include/asm/topology.h
+++ linux-2.6/arch/powerpc/include/asm/topology.h
@@ -18,12 +18,6 @@ struct device_node;
  */
 #define RECLAIM_DISTANCE 10
 
-/*
- * Avoid creating an extra level of balancing (SD_ALLNODES) on the largest
- * POWER7 boxes which have a maximum of 32 nodes.
- */
-#define SD_NODES_PER_DOMAIN 32
-
 #include <asm/mmzone.h>
 
 static inline int cpu_to_node(int cpu)
Index: linux-2.6/arch/x86/include/asm/topology.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/topology.h
+++ linux-2.6/arch/x86/include/asm/topology.h
@@ -92,44 +92,6 @@ extern void setup_node_to_cpumask_map(vo
 
 #define pcibus_to_node(bus) __pcibus_to_node(bus)
 
-#ifdef CONFIG_X86_32
-# define SD_CACHE_NICE_TRIES	1
-# define SD_IDLE_IDX		1
-#else
-# define SD_CACHE_NICE_TRIES	2
-# define SD_IDLE_IDX		2
-#endif
-
-/* sched_domains SD_NODE_INIT for NUMA machines */
-#define SD_NODE_INIT (struct sched_domain) {				\
-	.min_interval		= 8,					\
-	.max_interval		= 32,					\
-	.busy_factor		= 32,					\
-	.imbalance_pct		= 125,					\
-	.cache_nice_tries	= SD_CACHE_NICE_TRIES,			\
-	.busy_idx		= 3,					\
-	.idle_idx		= SD_IDLE_IDX,				\
-	.newidle_idx		= 0,					\
-	.wake_idx		= 0,					\
-	.forkexec_idx		= 0,					\
-									\
-	.flags			= 1*SD_LOAD_BALANCE			\
-				| 1*SD_BALANCE_NEWIDLE			\
-				| 1*SD_BALANCE_EXEC			\
-				| 1*SD_BALANCE_FORK			\
-				| 0*SD_BALANCE_WAKE			\
-				| 1*SD_WAKE_AFFINE			\
-				| 0*SD_PREFER_LOCAL			\
-				| 0*SD_SHARE_CPUPOWER			\
-				| 0*SD_POWERSAVINGS_BALANCE		\
-				| 0*SD_SHARE_PKG_RESOURCES		\
-				| 1*SD_SERIALIZE			\
-				| 0*SD_PREFER_SIBLING			\
-				,					\
-	.last_balance		= jiffies,				\
-	.balance_interval	= 1,					\
-}
-
 extern int __node_distance(int, int);
 #define node_distance(a, b) __node_distance(a, b)
 


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