[PATCH v3 2/2] powerpc: Enable CPU_FTR_ASYM_SMT for interleaved big-cores

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>

A pair of IBM POWER9 SMT4 cores can be fused together to form a big-core
with 8 SMT threads. This can be discovered via the "ibm,thread-groups"
CPU property in the device tree which will indicate which group of
threads that share the L1 cache, translation cache and instruction data
flow. If there are multiple such group of threads, then the core is a
big-core.

Furthermore, if the thread-ids of the threads of the big-core can be
obtained by interleaving the thread-ids of the thread-groups
(component small core), then such a big-core is called an interleaved
big-core.

Eg: Threads in the pair of component SMT4 cores of an interleaved
big-core are numbered {0,2,4,6} and {1,3,5,7} respectively.

The SMT4 cores forming a big-core are more or less independent
units. Thus when multiple tasks are scheduled to run on the fused
core, we get the best performance when the tasks are spread across the
pair of SMT4 cores.

This patch enables CPU_FTR_ASYM_SMT bit in the cpu-features on
detecting the presence of interleaved big-cores at boot up. This will
will bias the load-balancing of tasks on smaller numbered threads,
which will automatically result in spreading the tasks uniformly
across the associated pair of SMT4 cores.

Signed-off-by: Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/kernel/setup-common.c | 67 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 66 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
index 989edc1..c200fb8 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
@@ -581,6 +581,56 @@ int get_cpu_thread_group_start(int cpu, struct thread_groups *tg)
 	return -1;
 }
 
+/*
+ * check_interleaved_big_core - Checks if the thread group tg
+ * corresponds to a big-core whose threads are interleavings of the
+ * threads of the component small cores.
+ *
+ * @tg: A thread-group struct for the core.
+ *
+ * Returns true if the core is a interleaved big-core.
+ * Returns false otherwise.
+ */
+static inline bool check_interleaved_big_core(struct thread_groups *tg)
+{
+	int nr_groups;
+	int threads_per_group;
+	int cur_cpu, next_cpu, i, j;
+
+	nr_groups = tg->nr_groups;
+	threads_per_group = tg->threads_per_group;
+
+	if (tg->property != 1)
+		return false;
+
+	if (nr_groups < 2 || threads_per_group < 2)
+		return false;
+
+	/*
+	 * In case of an interleaved big-core, the thread-ids of the
+	 * big-core can be obtained by interleaving the the thread-ids
+	 * of the component small
+	 *
+	 * Eg: On a 8-thread big-core with two SMT4 small cores, the
+	 * threads of the two component small cores will be
+	 * {0, 2, 4, 6} and {1, 3, 5, 7}.
+	 */
+	for (i = 0; i < nr_groups; i++) {
+		int group_start = i * threads_per_group;
+
+		for (j = 0; j < threads_per_group - 1; j++) {
+			int cur_idx = group_start + j;
+
+			cur_cpu = tg->thread_list[cur_idx];
+			next_cpu = tg->thread_list[cur_idx + 1];
+			if (next_cpu != cur_cpu + nr_groups)
+				return false;
+		}
+	}
+
+	return true;
+}
+
 /**
  * setup_cpu_maps - initialize the following cpu maps:
  *                  cpu_possible_mask
@@ -604,6 +654,7 @@ void __init smp_setup_cpu_maps(void)
 	struct device_node *dn;
 	int cpu = 0;
 	int nthreads = 1;
+	bool has_interleaved_big_cores = true;
 
 	has_big_cores = true;
 	DBG("smp_setup_cpu_maps()\n");
@@ -657,6 +708,12 @@ void __init smp_setup_cpu_maps(void)
 
 		if (has_big_cores && !dt_has_big_core(dn, &tg)) {
 			has_big_cores = false;
+			has_interleaved_big_cores = false;
+		}
+
+		if (has_interleaved_big_cores) {
+			has_interleaved_big_cores =
+				check_interleaved_big_core(&tg);
 		}
 
 		if (cpu >= nr_cpu_ids) {
@@ -713,7 +770,15 @@ void __init smp_setup_cpu_maps(void)
 	vdso_data->processorCount = num_present_cpus();
 #endif /* CONFIG_PPC64 */
 
-        /* Initialize CPU <=> thread mapping/
+	if (has_interleaved_big_cores) {
+		int key = __builtin_ctzl(CPU_FTR_ASYM_SMT);
+
+		cur_cpu_spec->cpu_features |= CPU_FTR_ASYM_SMT;
+		static_branch_enable(&cpu_feature_keys[key]);
+		pr_info("Detected interleaved big-cores\n");
+	}
+
+	/* Initialize CPU <=> thread mapping/
 	 *
 	 * WARNING: We assume that the number of threads is the same for
 	 * every CPU in the system. If that is not the case, then some code
-- 
1.9.4