linux-kernel - [PATCH 3/9] cpu/SMT: Store the current/max number of threads

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Message-Id: <20230524155630.794584-3-mpe@ellerman.id.au>
Date:   Thu, 25 May 2023 01:56:24 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     <linux-kernel@...r.kernel.org>
Cc:     <linuxppc-dev@...ts.ozlabs.org>, <linux-arch@...r.kernel.org>,
        <ldufour@...ux.ibm.com>, <tglx@...utronix.de>, bp@...en8.de,
        dave.hansen@...ux.intel.com, mingo@...hat.com, x86@...nel.org
Subject: [PATCH 3/9] cpu/SMT: Store the current/max number of threads

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

To support that the SMT code needs to know the maximum number of SMT
threads, and also the currently configured number.

The arch code knows the max number of threads, so have the arch code
pass that value to cpu_smt_check_topology(). Note that although
topology_max_smt_threads() exists, it is not configured early enough to
be used here.

Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
---
 arch/x86/kernel/cpu/bugs.c |  3 ++-
 include/linux/cpu_smt.h    |  6 ++++--
 kernel/cpu.c               | 12 +++++++++++-
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 182af64387d0..3406799c1e9d 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -34,6 +34,7 @@
 #include <asm/hypervisor.h>
 #include <asm/tlbflush.h>
 #include <asm/cpu.h>
+#include <asm/smp.h>
 
 #include "cpu.h"
 
@@ -133,7 +134,7 @@ void __init check_bugs(void)
 	 * identify_boot_cpu() initialized SMT support information, let the
 	 * core code know.
 	 */
-	cpu_smt_check_topology();
+	cpu_smt_check_topology(smp_num_siblings);
 
 	if (!IS_ENABLED(CONFIG_SMP)) {
 		pr_info("CPU: ");
diff --git a/include/linux/cpu_smt.h b/include/linux/cpu_smt.h
index 17e105b52d85..8d4ae26047c9 100644
--- a/include/linux/cpu_smt.h
+++ b/include/linux/cpu_smt.h
@@ -12,15 +12,17 @@ enum cpuhp_smt_control {
 
 #if defined(CONFIG_SMP) && defined(CONFIG_HOTPLUG_SMT)
 extern enum cpuhp_smt_control cpu_smt_control;
+extern unsigned int cpu_smt_num_threads;
 extern void cpu_smt_disable(bool force);
-extern void cpu_smt_check_topology(void);
+extern void cpu_smt_check_topology(unsigned int num_threads);
 extern bool cpu_smt_possible(void);
 extern int cpuhp_smt_enable(void);
 extern int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval);
 #else
 # define cpu_smt_control		(CPU_SMT_NOT_IMPLEMENTED)
+# define cpu_smt_num_threads 1
 static inline void cpu_smt_disable(bool force) { }
-static inline void cpu_smt_check_topology(void) { }
+static inline void cpu_smt_check_topology(unsigned int num_threads) { }
 static inline bool cpu_smt_possible(void) { return false; }
 static inline int cpuhp_smt_enable(void) { return 0; }
 static inline int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) { return 0; }
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 01398ce3e131..a08dd8f93675 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -414,6 +414,8 @@ void __weak arch_smt_update(void) { }
 
 #ifdef CONFIG_HOTPLUG_SMT
 enum cpuhp_smt_control cpu_smt_control __read_mostly = CPU_SMT_ENABLED;
+static unsigned int cpu_smt_max_threads __ro_after_init;
+unsigned int cpu_smt_num_threads;
 
 void __init cpu_smt_disable(bool force)
 {
@@ -433,10 +435,18 @@ void __init cpu_smt_disable(bool force)
  * The decision whether SMT is supported can only be done after the full
  * CPU identification. Called from architecture code.
  */
-void __init cpu_smt_check_topology(void)
+void __init cpu_smt_check_topology(unsigned int num_threads)
 {
 	if (!topology_smt_supported())
 		cpu_smt_control = CPU_SMT_NOT_SUPPORTED;
+
+	cpu_smt_max_threads = num_threads;
+
+	// May already be disabled by nosmt command line parameter
+	if (cpu_smt_control != CPU_SMT_ENABLED)
+		cpu_smt_num_threads = 1;
+	else
+		cpu_smt_num_threads = num_threads;
 }
 
 static int __init smt_cmdline_disable(char *str)
-- 
2.40.1