[PATCH v4 1/5] x86/smp: Add a per-cpu view of SMT state

[Balbir Singh <sblbir@amazon.com>]

A new field smt_active in cpuinfo_x86 identifies if the current core/cpu
is in SMT mode or not. This can be very helpful if the system has some
of its cores with threads offlined and can be used for cases where
action is taken based on the state of SMT. The follow up patches use
this feature.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Balbir Singh <sblbir@amazon.com>
---
 arch/x86/include/asm/processor.h |  2 ++
 arch/x86/kernel/smpboot.c        | 10 +++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index c20a52b5534b..a411466a6e74 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -136,6 +136,8 @@ struct cpuinfo_x86 {
 	u16			logical_die_id;
 	/* Index into per_cpu list: */
 	u16			cpu_index;
+	/*  Is SMT active on this core? */
+	bool			smt_active;
 	u32			microcode;
 	/* Address space bits used by the cache internally */
 	u8			x86_cache_bits;
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 8ca66af96a54..5f6df298d785 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -640,6 +640,9 @@ void set_cpu_sibling_map(int cpu)
 	threads = cpumask_weight(topology_sibling_cpumask(cpu));
 	if (threads > __max_smt_threads)
 		__max_smt_threads = threads;
+
+	for_each_cpu(i, topology_sibling_cpumask(cpu))
+		cpu_data(i).smt_active = threads > 1;
 }
 
 /* maps the cpu to the sched domain representing multi-core */
@@ -1551,8 +1554,13 @@ static void remove_siblinginfo(int cpu)
 
 	for_each_cpu(sibling, topology_die_cpumask(cpu))
 		cpumask_clear_cpu(cpu, topology_die_cpumask(sibling));
-	for_each_cpu(sibling, topology_sibling_cpumask(cpu))
+
+	for_each_cpu(sibling, topology_sibling_cpumask(cpu)) {
 		cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
+		if (cpumask_weight(topology_sibling_cpumask(sibling)) == 1)
+			cpu_data(sibling).smt_active = false;
+	}
+
 	for_each_cpu(sibling, cpu_llc_shared_mask(cpu))
 		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
 	cpumask_clear(cpu_llc_shared_mask(cpu));
-- 
2.17.1