[tip: x86/pti] x86/smp: Add a per-cpu view of SMT state

[tip-bot2 for Balbir Singh]

The following commit has been merged into the x86/pti branch of tip:

Commit-ID:     0a260b1c5867863121b044afa8087d6b37e4fb7d
Gitweb:        https://git.kernel.org/tip/0a260b1c5867863121b044afa8087d6b37e4fb7d
Author:        Balbir Singh <sblbir@xxxxxxxxxx>
AuthorDate:    Wed, 29 Jul 2020 10:10:59 +10:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Wed, 16 Sep 2020 15:08:02 +02:00

x86/smp: Add a per-cpu view of SMT state

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@xxxxxxxxxxxxx>
Signed-off-by: Balbir Singh <sblbir@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200729001103.6450-2-sblbir@xxxxxxxxxx

---
 arch/x86/include/asm/processor.h |  2 ++
 arch/x86/kernel/smpboot.c        | 11 ++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 97143d8..d9eb20f 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 f5ef689..5fc7e0e 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -635,6 +635,9 @@ 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 */
@@ -1548,10 +1551,16 @@ 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));
 	cpumask_clear(topology_sibling_cpumask(cpu));
 	cpumask_clear(topology_core_cpumask(cpu));