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SubjectRe: [PATCH RFC tip/core/rcu 6/6] rcu: Reduce cache-miss initialization latencies for large systems
From
Date
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@chello.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@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: linux-alpha@vger.kernel.org
Cc: linux-ia64@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-sh@vger.kernel.org
Cc: Matt Turner <mattst88@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: sparclinux@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: x86@kernel.org
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.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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