[PATCH 5/9] cpu/SMT: Create topology_smt_thread_allowed()

Michael Ellerman
2023-05-24T15:59:14Z

A subsequent patch will enable partial SMT states, ie. when not all SMT
threads are brought online.

To support that, add an arch helper which checks whether a given CPU is
allowed to be brought online depending on how many SMT threads are
currently enabled.

Call the helper from cpu_smt_enable(), and cpu_smt_allowed() when SMT is
enabled, to check if the particular thread should be onlined. Notably,
also call it from cpu_smt_disable() if CPU_SMT_ENABLED, to allow
offlining some threads to move from a higher to lower number of threads
online.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
arch/x86/include/asm/topology.h | 2 ++
arch/x86/kernel/smpboot.c | 15 +++++++++++++++
kernel/cpu.c | 10 +++++++++-
3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index 197ec2589f5d..c6245ea6e589 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -145,6 +145,7 @@ int topology_phys_to_logical_die(unsigned int die, unsigned int cpu);
bool topology_is_primary_thread(unsigned int cpu);
bool topology_smt_supported(void);
bool topology_smt_threads_supported(unsigned int threads);
+bool topology_smt_thread_allowed(unsigned int cpu);
#else
#define topology_max_packages() (1)
static inline int
@@ -159,6 +160,7 @@ static inline int topology_max_smt_threads(void) { return 1; }
static inline bool topology_is_primary_thread(unsigned int cpu) { return true; }
static inline bool topology_smt_supported(void) { return false; }
static inline bool topology_smt_threads_supported(unsigned int threads) { return false; }
+static inline bool topology_smt_thread_allowed(unsigned int cpu) { return false; }
#endif

static inline void arch_fix_phys_package_id(int num, u32 slot)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index c7ba62beae3e..244b4df40600 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -298,6 +298,21 @@ bool topology_smt_threads_supported(unsigned int threads)
return threads == 1 || threads == smp_num_siblings;
}

+/**
+ * topology_smt_thread_allowed - When enabling SMT check whether this particular
+ * CPU thread is allowed to be brought online.
+ * @cpu: CPU to check
+ */
+bool topology_smt_thread_allowed(unsigned int cpu)
+{
+ /*
+ * No extra logic s required here to support different thread values
+ * because threads will always == 1 or smp_num_siblings because of
+ * topology_smt_threads_supported().
+ */
+ return true;
+}
+
/**
* topology_phys_to_logical_pkg - Map a physical package id to a logical
*
diff --git a/kernel/cpu.c b/kernel/cpu.c
index a08dd8f93675..72b9a03a4fef 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -458,7 +458,7 @@ early_param("nosmt", smt_cmdline_disable);

static inline bool cpu_smt_allowed(unsigned int cpu)
{
- if (cpu_smt_control == CPU_SMT_ENABLED)
+ if (cpu_smt_control == CPU_SMT_ENABLED && topology_smt_thread_allowed(cpu))
return true;

if (topology_is_primary_thread(cpu))
@@ -2273,6 +2273,12 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
for_each_online_cpu(cpu) {
if (topology_is_primary_thread(cpu))
continue;
+ /*
+ * Disable can be called with CPU_SMT_ENABLED when changing
+ * from a higher to lower number of SMT threads per core.
+ */
+ if (ctrlval == CPU_SMT_ENABLED && topology_smt_thread_allowed(cpu))
+ continue;
ret = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
if (ret)
break;
@@ -2307,6 +2313,8 @@ int cpuhp_smt_enable(void)
/* Skip online CPUs and CPUs on offline nodes */
if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
continue;
+ if (!topology_smt_thread_allowed(cpu))
+ continue;
ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
if (ret)
break;
--
2.40.1