cpu/SMT: Enable SMT only if a core is online

[ Upstream commit 6c17ea1f3eaa330d445ac14a9428402ce4e3055e ]

If a core is offline then enabling SMT should not online CPUs of
this core. By enabling SMT, what is intended is either changing the SMT
value from "off" to "on" or setting the SMT level (threads per core) from a
lower to higher value.

On PowerPC the ppc64_cpu utility can be used, among other things, to
perform the following functions:

ppc64_cpu --cores-on                # Get the number of online cores
ppc64_cpu --cores-on=X              # Put exactly X cores online
ppc64_cpu --offline-cores=X[,Y,...] # Put specified cores offline
ppc64_cpu --smt={on|off|value}      # Enable, disable or change SMT level

If the user has decided to offline certain cores, enabling SMT should
not online CPUs in those cores. This patch fixes the issue and changes
the behaviour as described, by introducing an arch specific function
topology_is_core_online(). It is currently implemented only for PowerPC.

Fixes: 73c58e7e1412 ("powerpc: Add HOTPLUG_SMT support")
Reported-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Closes: https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
Signed-off-by: Nysal Jan K.A <nysal@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240731030126.956210-2-nysal@linux.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index e7e160954e798e50ecd5bea999b03e31bf280d7f..0579860b55299804b42c84e21f628204caef2299 100644 (file)
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -562,7 +562,8 @@ Description:        Control Symmetric Multi Threading (SMT)
                         ================ =========================================
 
                         If control status is "forceoff" or "notsupported" writes
-                        are rejected.
+                        are rejected. Note that enabling SMT on PowerPC skips
+                        offline cores.
 
 What:          /sys/devices/system/cpu/cpuX/power/energy_perf_bias
 Date:          March 2019

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 3d2bf1d50a0c4a5608a48173ee9deea3758b4b50..6dee328bfe6fddb6a0625649460999643f8c7b49 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2679,6 +2679,16 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
        return ret;
 }
 
+/**
+ * Check if the core a CPU belongs to is online
+ */
+#if !defined(topology_is_core_online)
+static inline bool topology_is_core_online(unsigned int cpu)
+{
+       return true;
+}
+#endif
+
 int cpuhp_smt_enable(void)
 {
        int cpu, ret = 0;
@@ -2689,7 +2699,7 @@ 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 (!cpu_smt_thread_allowed(cpu))
+               if (!cpu_smt_thread_allowed(cpu) || !topology_is_core_online(cpu))
                        continue;
                ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
                if (ret)