[PATCH v9 0/3] powerpc: Detection and scheduler optimization for POWER9 bigcore

[PATCH v9 0/3] powerpc: Detection and scheduler optimization for POWER9 bigcore

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>

Hi,

This is the ninth iteration of the patchset to add support for
big-core on POWER9. This patch also optimizes the task placement on
such big-core systems.

The previous versions can be found here:
v8: https://lkml.org/lkml/2018/9/20/899
v7: https://lkml.org/lkml/2018/8/20/52
v6: https://lkml.org/lkml/2018/8/9/119
v5: https://lkml.org/lkml/2018/8/6/587
v4: https://lkml.org/lkml/2018/7/24/79
v3: https://lkml.org/lkml/2018/7/6/255
v2: https://lkml.org/lkml/2018/7/3/401
v1: https://lkml.org/lkml/2018/5/11/245

Changes :
v8 --> v9:
   - Rebased it on v4.19-rc5
   - Updated the commit log for the second patch as per Dave Hansen's
   suggestion.
   - Fixed the build errors reported by Michael Neuling and the Kernel
   Build bot.

Description:
~~~~~~~~~~~~~~~~~~~~

IBM POWER9 SMT8 cores consists of two groups of small-cores where each
group has its own L1 cache, translation cache and instruction-data
flow. This can be discovered via the "ibm,thread-groups" CPU property
in the device tree. Furthermore, on POWER9 the thread-ids of such a
big-core is obtained by interleaving the thread-ids of the two
small-cores.

Eg: In an SMT8 core with thread ids {0,1,2,3,4,5,6,7}, the thread-ids
of the threads in the two small-cores respectively will be {0,2,4,6}
and {1,3,5,7} respectively.

 	   -------------------------
	   |  	    L1 Cache       |
       ----------------------------------
       |L2|     |     |     |      |
       |  |  0  |  2  |  4  |  6   |Small Core0
       |C |     |     |     |      |
Big    |a --------------------------
Core   |c |     |     |     |      |
       |h |  1  |  3  |  5  |  7   | Small Core1
       |e |     |     |     |      |
       -----------------------------
	  |  	    L1 Cache       |
	  --------------------------

On such a big-core system, when multiple tasks are scheduled to run on
the big-core, we get the best performance when the tasks are spread
across the pair of small-cores.

Eg: Suppose there 4 tasks {p1, p2, p3, p4} are run on a big core, then

An Example of Optimal Task placement:
	   --------------------------
           |     |     |     |      |
           |  0  |  2  |  4  |  6   |   Small Core0
           | (p1)| (p2)|     |      |
Big Core   --------------------------
           |     |     |     |      |
           |  1  |  3  |  5  |  7   |   Small Core1
           |     | (p3)|     | (p4) |
           --------------------------

An example of Suboptimal Task placement:
	   --------------------------
           |     |     |     |      |
           |  0  |  2  |  4  |  6   |   Small Core0
           | (p1)| (p2)|     |  (p4)|
Big Core   --------------------------
           |     |     |     |      |
           |  1  |  3  |  5  |  7   |   Small Core1
           |     | (p3)|     |      |
           --------------------------

Currently on the big-core systems, the sched domain hierarchy is:

SMT   : group of CPUs in the SMT8 core.
DIE   : groups of CPUs on the same die.
NUMA  : all the CPUs in the system.

Thus the scheduler doesn't distinguish between CPUs in the core that
share the L1-cache vs the ones that don't resulting in a run-to-run
variance when multithreaded applications are run on an SMT8 core.

In this patch-set, we address this by defining the sched-domain on the
big-core systems to be:

SMT   : group of CPUs sharing the L1 cache
CACHE : group of CPUs in the SMT8 core.
DIE   : groups of CPUs on the same die.
NUMA  : all the CPUs in the system.

With this, the Linux Kernel load-balancer will ensure that the tasks
are spread across all the component small cores in the system, thereby
yielding optimum performance.

Furthermore, this solution works correctly across all SMT modes
(8,4,2), as the interleaved thread-ids ensures that when we go to
lower SMT modes (4,2) the threads are offlined in a descending order,
thereby leaving equal number of threads from the component small cores
online as illustrated below.

This patchset contains three patches which on detecting the presence
of big-cores, defines the SMT level sched domain to correspond to the
threads of the small cores.

Patch 1: adds support to detect the presence of
big-cores and parses the output of "ibm,thread-groups" device-tree
which using which it updates a per-cpu mask named cpu_smallcore_mask

Patch 2: Defines the SMT level sched domain to correspond to the
threads of the small cores.

Patch 3: Creates a pair of sysfs attributes named
	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings
	  and
	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings_list
	  exposing the small-core siblings that share the L1 cache
	  to the userspace.

Results:
~~~~~~~~~~~~~~~~~
1) 2 thread ebizzy
~~~~~~~~~~~~~~~~~~~~~~
Experimental results for ebizzy with 2 threads, bound to a single big-core
show a marked improvement with this patchset over the 4.19.0-rc5 vanilla
kernel.

The result of 100 such runs for 4.19.0-rc5 kernel and the
4.19.0-rc5 + big-core-patches are as follows

4.19.0-rc5 vanilla
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        records/s    :  # samples  : Histogram
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[0 -       1000000]  :      0      : #
[1000000 - 2000000]  :      1      : #
[2000000 - 3000000]  :      8      : ##
[3000000 - 4000000]  :      31     : #######
[4000000 - 5000000]  :      5      : ##
[5000000 - 6000000]  :      2      : #
[6000000 - 7000000]  :      53     : ###########
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

4.19.0-rc5 + big-core-patches
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        records/s    :  # samples  : Histogram
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[0 -       1000000]  :      0      : #
[1000000 - 2000000]  :      0      : #
[2000000 - 3000000]  :      7      : ##
[3000000 - 4000000]  :      8      : ##
[4000000 - 5000000]  :      0      : #
[5000000 - 6000000]  :      1      : #
[6000000 - 7000000]  :      84     : #################
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2) Hackbench (perf bench sched pipe)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

500 iterations of the hackbench run both on 4.19.0-rc5 vanilla kernel
and v4.19.0-rc5 + big-core-patches. There isn't a significant
difference between the two.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
			4.19.0-rc5 vanilla
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    N           Min           Max        Median           Avg        Stddev
  500         5.195         7.108         6.818      6.634932    0.39956043
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
			4.19.0-rc5 + big-core-patches
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    N           Min           Max        Median           Avg        Stddev
  500         5.215         6.978         6.895      6.759908    0.33699356
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Gautham R. Shenoy (3):
  powerpc: Detect the presence of big-cores via "ibm,thread-groups"
  powerpc: Use cpu_smallcore_sibling_mask at SMT level on bigcores
  powerpc/sysfs: Add topology/smallcore_thread_siblings[_list]

 Documentation/ABI/testing/sysfs-devices-system-cpu |  14 ++
 arch/powerpc/include/asm/cputhreads.h              |   2 +
 arch/powerpc/include/asm/smp.h                     |   6 +
 arch/powerpc/kernel/smp.c                          | 241 ++++++++++++++++++++-
 arch/powerpc/kernel/sysfs.c                        |  91 ++++++++
 5 files changed, 353 insertions(+), 1 deletion(-)

-- 
1.9.4

[PATCH v9 1/3] powerpc: Detect the presence of big-cores via "ibm,thread-groups"

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>

On IBM POWER9, the device tree exposes a property array identifed by
"ibm,thread-groups" which will indicate which groups of threads share
a particular set of resources.

As of today we only have one form of grouping identifying the group of
threads in the core that share the L1 cache, translation cache and
instruction data flow.

This patch adds helper functions to parse the contents of
"ibm,thread-groups" and populate a per-cpu variable to cache
information about siblings of each CPU that share the L1, traslation
cache and instruction data-flow.

It also defines a new global variable named "has_big_cores" which
indicates if the cores on this configuration have multiple groups of
threads that share L1 cache.

For each online CPU, it maintains a cpu_smallcore_mask, which
indicates the online siblings which share the L1-cache with it.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/cputhreads.h |   2 +
 arch/powerpc/include/asm/smp.h        |   6 +
 arch/powerpc/kernel/smp.c             | 222 ++++++++++++++++++++++++++++++++++
 3 files changed, 230 insertions(+)

diff --git a/arch/powerpc/include/asm/cputhreads.h b/arch/powerpc/include/asm/cputhreads.h
index d71a909..deb99fd 100644
--- a/arch/powerpc/include/asm/cputhreads.h
+++ b/arch/powerpc/include/asm/cputhreads.h
@@ -23,11 +23,13 @@
 extern int threads_per_core;
 extern int threads_per_subcore;
 extern int threads_shift;
+extern bool has_big_cores;
 extern cpumask_t threads_core_mask;
 #else
 #define threads_per_core	1
 #define threads_per_subcore	1
 #define threads_shift		0
+#define has_big_cores		0
 #define threads_core_mask	(*get_cpu_mask(0))
 #endif
 
diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
index 95b66a0..4439893 100644
--- a/arch/powerpc/include/asm/smp.h
+++ b/arch/powerpc/include/asm/smp.h
@@ -100,6 +100,7 @@ static inline void set_hard_smp_processor_id(int cpu, int phys)
 DECLARE_PER_CPU(cpumask_var_t, cpu_sibling_map);
 DECLARE_PER_CPU(cpumask_var_t, cpu_l2_cache_map);
 DECLARE_PER_CPU(cpumask_var_t, cpu_core_map);
+DECLARE_PER_CPU(cpumask_var_t, cpu_smallcore_map);
 
 static inline struct cpumask *cpu_sibling_mask(int cpu)
 {
@@ -116,6 +117,11 @@ static inline struct cpumask *cpu_l2_cache_mask(int cpu)
 	return per_cpu(cpu_l2_cache_map, cpu);
 }
 
+static inline struct cpumask *cpu_smallcore_mask(int cpu)
+{
+	return per_cpu(cpu_smallcore_map, cpu);
+}
+
 extern int cpu_to_core_id(int cpu);
 
 /* Since OpenPIC has only 4 IPIs, we use slightly different message numbers.
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 61c1fad..22a14a9 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -74,14 +74,32 @@
 #endif
 
 struct thread_info *secondary_ti;
+bool has_big_cores;
 
 DEFINE_PER_CPU(cpumask_var_t, cpu_sibling_map);
+DEFINE_PER_CPU(cpumask_var_t, cpu_smallcore_map);
 DEFINE_PER_CPU(cpumask_var_t, cpu_l2_cache_map);
 DEFINE_PER_CPU(cpumask_var_t, cpu_core_map);
 
 EXPORT_PER_CPU_SYMBOL(cpu_sibling_map);
 EXPORT_PER_CPU_SYMBOL(cpu_l2_cache_map);
 EXPORT_PER_CPU_SYMBOL(cpu_core_map);
+EXPORT_SYMBOL_GPL(has_big_cores);
+
+#define MAX_THREAD_LIST_SIZE	8
+#define THREAD_GROUP_SHARE_L1   1
+struct thread_groups {
+	unsigned int property;
+	unsigned int nr_groups;
+	unsigned int threads_per_group;
+	unsigned int thread_list[MAX_THREAD_LIST_SIZE];
+};
+
+/*
+ * On big-cores system, cpu_l1_cache_map for each CPU corresponds to
+ * the set its siblings that share the L1-cache.
+ */
+DEFINE_PER_CPU(cpumask_var_t, cpu_l1_cache_map);
 
 /* SMP operations for this machine */
 struct smp_ops_t *smp_ops;
@@ -674,6 +692,185 @@ static void set_cpus_unrelated(int i, int j,
 }
 #endif
 
+/*
+ * parse_thread_groups: Parses the "ibm,thread-groups" device tree
+ *                      property for the CPU device node @dn and stores
+ *                      the parsed output in the thread_groups
+ *                      structure @tg if the ibm,thread-groups[0]
+ *                      matches @property.
+ *
+ * @dn: The device node of the CPU device.
+ * @tg: Pointer to a thread group structure into which the parsed
+ *      output of "ibm,thread-groups" is stored.
+ * @property: The property of the thread-group that the caller is
+ *            interested in.
+ *
+ * ibm,thread-groups[0..N-1] array defines which group of threads in
+ * the CPU-device node can be grouped together based on the property.
+ *
+ * ibm,thread-groups[0] tells us the property based on which the
+ * threads are being grouped together. If this value is 1, it implies
+ * that the threads in the same group share L1, translation cache.
+ *
+ * ibm,thread-groups[1] tells us how many such thread groups exist.
+ *
+ * ibm,thread-groups[2] tells us the number of threads in each such
+ * group.
+ *
+ * ibm,thread-groups[3..N-1] is the list of threads identified by
+ * "ibm,ppc-interrupt-server#s" arranged as per their membership in
+ * the grouping.
+ *
+ * Example: If ibm,thread-groups = [1,2,4,5,6,7,8,9,10,11,12] it
+ * implies that there are 2 groups of 4 threads each, where each group
+ * of threads share L1, translation cache.
+ *
+ * The "ibm,ppc-interrupt-server#s" of the first group is {5,6,7,8}
+ * and the "ibm,ppc-interrupt-server#s" of the second group is {9, 10,
+ * 11, 12} structure
+ *
+ * Returns 0 on success, -EINVAL if the property does not exist,
+ * -ENODATA if property does not have a value, and -EOVERFLOW if the
+ * property data isn't large enough.
+ */
+static int parse_thread_groups(struct device_node *dn,
+			       struct thread_groups *tg,
+			       unsigned int property)
+{
+	int i;
+	u32 thread_group_array[3 + MAX_THREAD_LIST_SIZE];
+	u32 *thread_list;
+	size_t total_threads;
+	int ret;
+
+	ret = of_property_read_u32_array(dn, "ibm,thread-groups",
+					 thread_group_array, 3);
+	if (ret)
+		return ret;
+
+	tg->property = thread_group_array[0];
+	tg->nr_groups = thread_group_array[1];
+	tg->threads_per_group = thread_group_array[2];
+	if (tg->property != property ||
+	    tg->nr_groups < 1 ||
+	    tg->threads_per_group < 1)
+		return -ENODATA;
+
+	total_threads = tg->nr_groups * tg->threads_per_group;
+
+	ret = of_property_read_u32_array(dn, "ibm,thread-groups",
+					 thread_group_array,
+					 3 + total_threads);
+	if (ret)
+		return ret;
+
+	thread_list = &thread_group_array[3];
+
+	for (i = 0 ; i < total_threads; i++)
+		tg->thread_list[i] = thread_list[i];
+
+	return 0;
+}
+
+/*
+ * get_cpu_thread_group_start : Searches the thread group in tg->thread_list
+ *                              that @cpu belongs to.
+ *
+ * @cpu : The logical CPU whose thread group is being searched.
+ * @tg : The thread-group structure of the CPU node which @cpu belongs
+ *       to.
+ *
+ * Returns the index to tg->thread_list that points to the the start
+ * of the thread_group that @cpu belongs to.
+ *
+ * Returns -1 if cpu doesn't belong to any of the groups pointed to by
+ * tg->thread_list.
+ */
+static int get_cpu_thread_group_start(int cpu, struct thread_groups *tg)
+{
+	int hw_cpu_id = get_hard_smp_processor_id(cpu);
+	int i, j;
+
+	for (i = 0; i < tg->nr_groups; i++) {
+		int group_start = i * tg->threads_per_group;
+
+		for (j = 0; j < tg->threads_per_group; j++) {
+			int idx = group_start + j;
+
+			if (tg->thread_list[idx] == hw_cpu_id)
+				return group_start;
+		}
+	}
+
+	return -1;
+}
+
+static int init_cpu_l1_cache_map(int cpu)
+
+{
+	struct device_node *dn = of_get_cpu_node(cpu, NULL);
+	struct thread_groups tg = {.property = 0,
+				   .nr_groups = 0,
+				   .threads_per_group = 0};
+	int first_thread = cpu_first_thread_sibling(cpu);
+	int i, cpu_group_start = -1, err = 0;
+
+	if (!dn)
+		return -ENODATA;
+
+	err = parse_thread_groups(dn, &tg, THREAD_GROUP_SHARE_L1);
+	if (err)
+		goto out;
+
+	zalloc_cpumask_var_node(&per_cpu(cpu_l1_cache_map, cpu),
+				GFP_KERNEL,
+				cpu_to_node(cpu));
+
+	cpu_group_start = get_cpu_thread_group_start(cpu, &tg);
+
+	if (unlikely(cpu_group_start == -1)) {
+		WARN_ON_ONCE(1);
+		err = -ENODATA;
+		goto out;
+	}
+
+	for (i = first_thread; i < first_thread + threads_per_core; i++) {
+		int i_group_start = get_cpu_thread_group_start(i, &tg);
+
+		if (unlikely(i_group_start == -1)) {
+			WARN_ON_ONCE(1);
+			err = -ENODATA;
+			goto out;
+		}
+
+		if (i_group_start == cpu_group_start)
+			cpumask_set_cpu(i, per_cpu(cpu_l1_cache_map, cpu));
+	}
+
+out:
+	of_node_put(dn);
+	return err;
+}
+
+static int init_big_cores(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		int err = init_cpu_l1_cache_map(cpu);
+
+		if (err)
+			return err;
+
+		zalloc_cpumask_var_node(&per_cpu(cpu_smallcore_map, cpu),
+					GFP_KERNEL,
+					cpu_to_node(cpu));
+	}
+
+	has_big_cores = true;
+	return 0;
+}
+
 void __init smp_prepare_cpus(unsigned int max_cpus)
 {
 	unsigned int cpu;
@@ -712,6 +909,12 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 	cpumask_set_cpu(boot_cpuid, cpu_l2_cache_mask(boot_cpuid));
 	cpumask_set_cpu(boot_cpuid, cpu_core_mask(boot_cpuid));
 
+	init_big_cores();
+	if (has_big_cores) {
+		cpumask_set_cpu(boot_cpuid,
+				cpu_smallcore_mask(boot_cpuid));
+	}
+
 	if (smp_ops && smp_ops->probe)
 		smp_ops->probe();
 }
@@ -995,10 +1198,28 @@ static void remove_cpu_from_masks(int cpu)
 		set_cpus_unrelated(cpu, i, cpu_core_mask);
 		set_cpus_unrelated(cpu, i, cpu_l2_cache_mask);
 		set_cpus_unrelated(cpu, i, cpu_sibling_mask);
+		if (has_big_cores)
+			set_cpus_unrelated(cpu, i, cpu_smallcore_mask);
 	}
 }
 #endif
 
+static inline void add_cpu_to_smallcore_masks(int cpu)
+{
+	struct cpumask *this_l1_cache_map = per_cpu(cpu_l1_cache_map, cpu);
+	int i, first_thread = cpu_first_thread_sibling(cpu);
+
+	if (!has_big_cores)
+		return;
+
+	cpumask_set_cpu(cpu, cpu_smallcore_mask(cpu));
+
+	for (i = first_thread; i < first_thread + threads_per_core; i++) {
+		if (cpu_online(i) && cpumask_test_cpu(i, this_l1_cache_map))
+			set_cpus_related(i, cpu, cpu_smallcore_mask);
+	}
+}
+
 static void add_cpu_to_masks(int cpu)
 {
 	int first_thread = cpu_first_thread_sibling(cpu);
@@ -1015,6 +1236,7 @@ static void add_cpu_to_masks(int cpu)
 		if (cpu_online(i))
 			set_cpus_related(i, cpu, cpu_sibling_mask);
 
+	add_cpu_to_smallcore_masks(cpu);
 	/*
 	 * Copy the thread sibling mask into the cache sibling mask
 	 * and mark any CPUs that share an L2 with this CPU.
-- 
1.9.4

[PATCH v9 2/3] powerpc: Use cpu_smallcore_sibling_mask at SMT level on bigcores

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>

POWER9 SMT8 cores consist of two groups of threads, where threads in
each group shares L1-cache. The scheduler is not aware of this
distinction as the current sched-domain hierarchy has all the threads
of the core defined at the SMT domain.

	SMT  [Thread siblings of the SMT8 core]
	DIE  [CPUs in the same die]
	NUMA [All the CPUs in the system]

Due to this, we can observe run-to-run variance when we run a
multi-threaded benchmark bound to a single core based on how the
scheduler spreads the software threads across the two groups in the
core.

We fix this in this patch by defining each group of threads which
share L1-cache to be the SMT level. The group of threads in the SMT8
core is defined to be the CACHE level. The sched-domain hierarchy
after this patch will be :

	SMT	[Thread siblings in the core that share L1 cache]
	CACHE 	[Thread siblings that are in the SMT8 core]
	DIE  	[CPUs in the same die]
	NUMA 	[All the CPUs in the system]

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/smp.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 22a14a9..356751e 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1266,6 +1266,7 @@ static void add_cpu_to_masks(int cpu)
 void start_secondary(void *unused)
 {
 	unsigned int cpu = smp_processor_id();
+	struct cpumask *(*sibling_mask)(int) = cpu_sibling_mask;
 
 	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
@@ -1291,11 +1292,13 @@ void start_secondary(void *unused)
 	/* Update topology CPU masks */
 	add_cpu_to_masks(cpu);
 
+	if (has_big_cores)
+		sibling_mask = cpu_smallcore_mask;
 	/*
 	 * Check for any shared caches. Note that this must be done on a
 	 * per-core basis because one core in the pair might be disabled.
 	 */
-	if (!cpumask_equal(cpu_l2_cache_mask(cpu), cpu_sibling_mask(cpu)))
+	if (!cpumask_equal(cpu_l2_cache_mask(cpu), sibling_mask(cpu)))
 		shared_caches = true;
 
 	set_numa_node(numa_cpu_lookup_table[cpu]);
@@ -1362,6 +1365,13 @@ static const struct cpumask *shared_cache_mask(int cpu)
 	return cpu_l2_cache_mask(cpu);
 }
 
+#ifdef CONFIG_SCHED_SMT
+static const struct cpumask *smallcore_smt_mask(int cpu)
+{
+	return cpu_smallcore_mask(cpu);
+}
+#endif
+
 static struct sched_domain_topology_level power9_topology[] = {
 #ifdef CONFIG_SCHED_SMT
 	{ cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
@@ -1389,6 +1399,13 @@ void __init smp_cpus_done(unsigned int max_cpus)
 	shared_proc_topology_init();
 	dump_numa_cpu_topology();
 
+#ifdef CONFIG_SCHED_SMT
+	if (has_big_cores) {
+		pr_info("Using small cores at SMT level\n");
+		power9_topology[0].mask = smallcore_smt_mask;
+		powerpc_topology[0].mask = smallcore_smt_mask;
+	}
+#endif
 	/*
 	 * If any CPU detects that it's sharing a cache with another CPU then
 	 * use the deeper topology that is aware of this sharing.
-- 
1.9.4

[PATCH v9 3/3] powerpc/sysfs: Add topology/smallcore_thread_siblings[_list]

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>

This patch adds two sysfs attributes named smallcore_thread_siblings
and smallcore_thread_siblings_list to the "topology" attribute group
for each CPU device.

The read-only attributes
/sys/device/system/cpu/cpuN/topology/smallcore_thread_siblings and
/sys/device/system/cpu/cpuN/topology/smallcore_thread_siblings_list
will the online siblings of CPU N that share the L1 cache with it on
big-core configurations in cpumask format and cpu-list format
respectively.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
---
 Documentation/ABI/testing/sysfs-devices-system-cpu | 14 ++++
 arch/powerpc/kernel/sysfs.c                        | 91 ++++++++++++++++++++++
 2 files changed, 105 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 7331822..2a80dc2 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -511,3 +511,17 @@ Description:	Control Symetric Multi Threading (SMT)
 
 			 If control status is "forceoff" or "notsupported" writes
 			 are rejected.
+
+What:		/sys/devices/system/cpu/cpu#/topology/smallcore_thread_siblings
+		/sys/devices/system/cpu/cpu#/topology/smallcore_thread_siblings_list
+Date:		Sept 2018
+Contact:	Linux for PowerPC mailing list <linuxppc-dev@ozlabs.org>
+Description: 	CPU topology files that describe the thread siblings of a
+		logical CPU that share the L1-cache with it on POWER9
+		big-core configurations.
+
+		smallcore_thread_siblings: internal kernel map of
+		cpu#'s hardware threads that share L1-cache with cpu#.
+
+		smallcore_thread_siblings_list: human-readable list of
+		cpu#'s hardware threads that share L1-cache with cpu#.
diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
index 755dc98..3049511 100644
--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c
@@ -18,6 +18,7 @@
 #include <asm/smp.h>
 #include <asm/pmc.h>
 #include <asm/firmware.h>
+#include <asm/cputhreads.h>
 
 #include "cacheinfo.h"
 #include "setup.h"
@@ -1060,3 +1061,93 @@ static int __init topology_init(void)
 	return 0;
 }
 subsys_initcall(topology_init);
+
+#ifdef CONFIG_SMP
+static ssize_t smallcore_thread_siblings_show(struct device *dev,
+					      struct device_attribute *attr,
+					      char *buf)
+{
+	int cpu = dev->id;
+
+	return cpumap_print_to_pagebuf(false, buf, cpu_smallcore_mask(cpu));
+}
+static DEVICE_ATTR_RO(smallcore_thread_siblings);
+
+static ssize_t
+	smallcore_thread_siblings_list_show(struct device *dev,
+					    struct device_attribute *attr,
+					    char *buf)
+{
+	int cpu = dev->id;
+
+	return cpumap_print_to_pagebuf(true, buf, cpu_smallcore_mask(cpu));
+}
+static DEVICE_ATTR_RO(smallcore_thread_siblings_list);
+
+static struct attribute *smallcore_attrs[] = {
+	&dev_attr_smallcore_thread_siblings.attr,
+	&dev_attr_smallcore_thread_siblings_list.attr,
+	NULL
+};
+
+static const struct attribute_group smallcore_attr_group = {
+	.name = "topology",
+	.attrs = smallcore_attrs
+};
+
+static int smallcore_register_cpu_online(unsigned int cpu)
+{
+	int err;
+	struct device *cpu_dev = get_cpu_device(cpu);
+
+	if (!has_big_cores)
+		return 0;
+
+	err = sysfs_merge_group(&cpu_dev->kobj, &smallcore_attr_group);
+
+	return err;
+}
+
+static int smallcore_unregister_cpu_online(unsigned int cpu)
+{
+	struct device *cpu_dev = get_cpu_device(cpu);
+
+	if (!has_big_cores)
+		return 0;
+
+	sysfs_unmerge_group(&cpu_dev->kobj, &smallcore_attr_group);
+
+	return 0;
+}
+
+/*
+ * NOTE: The smallcore_register_cpu_online
+ *       (resp. smallcore_unregister_cpu_online) callback will merge
+ *       (resp. unmerge) a couple of additional attributes to the
+ *       "topology" attribute group of a CPU device when the CPU comes
+ *       online (resp. goes offline).
+ *
+ *       Hence, the registration of these callbacks must happen after
+ *       topology_sysfs_init() is called so that the topology
+ *       attribute group is created before these additional attributes
+ *       can be merged/unmerged. We cannot register these callbacks in
+ *       topology_init() since this function is called before
+ *       topology_sysfs_init(). Hence we define the following
+ *       late_initcall for this purpose.
+ */
+static int __init smallcore_topology_init(void)
+{
+	int r;
+
+	if (!has_big_cores)
+		return 0;
+
+	r = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
+			      "powerpc/topology/smallcore:online",
+			      smallcore_register_cpu_online,
+			      smallcore_unregister_cpu_online);
+	WARN_ON(r < 0);
+	return 0;
+}
+late_initcall(smallcore_topology_init);
+#endif /* CONFIG_SMP */
-- 
1.9.4

Re: [PATCH v9 0/3] powerpc: Detection and scheduler optimization for POWER9 bigcore

[Dave Hansen]

On 10/01/2018 06:16 AM, Gautham R. Shenoy wrote:
> 
> Patch 3: Creates a pair of sysfs attributes named
> 	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings
> 	  and
> 	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings_list
> 	  exposing the small-core siblings that share the L1 cache
> 	  to the userspace.

I really don't think you've justified the existence of a new user/kernel
interface here.  We already have information about threads share L1
caches in here:

	/sys/devices/system/cpu/cpu0/cache/index0/shared_cpu_list

The only question would be if anything would break because it assumes
that all SMT siblings share all caches.  But, it breaks if your new
interface is there or not; it's old software that we care about.

Re: [PATCH v9 0/3] powerpc: Detection and scheduler optimization for POWER9 bigcore

[Gautham R Shenoy]

Hello Dave,

On Mon, Oct 01, 2018 at 07:05:11AM -0700, Dave Hansen wrote:
> On 10/01/2018 06:16 AM, Gautham R. Shenoy wrote:
> > 
> > Patch 3: Creates a pair of sysfs attributes named
> > 	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings
> > 	  and
> > 	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings_list
> > 	  exposing the small-core siblings that share the L1 cache
> > 	  to the userspace.
> 
> I really don't think you've justified the existence of a new user/kernel
> interface here.  We already have information about threads share L1
> caches in here:
> 
> 	/sys/devices/system/cpu/cpu0/cache/index0/shared_cpu_list


Hmmm. My bad, I wasn't aware of this sysfs interface. We can use this
to share information about L1 cache. And currently on the upstream
kernel we report incorrect value on POWER9 SMT8 core. I will fix
this. Thanks for calling this out.

> 
> The only question would be if anything would break because it assumes
> that all SMT siblings share all caches.  But, it breaks if your new
> interface is there or not; it's old software that we care about.
>

Up until POWER9 SMT8 cores, this assumption was true that all SMT
siblings share all caches. POWER9 SMT8 cores are the only exception. 

--
Thanks and Regards
gautham.