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

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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: 73c58e7e14 ("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
This commit is contained in:
Nysal Jan K.A 2024-07-31 08:31:12 +05:30 committed by Michael Ellerman
parent e7e846dc6c
commit 6c17ea1f3e
2 changed files with 13 additions and 2 deletions
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3
Documentation/ABI/testing/sysfs-devices-system-cpu
View file

@ -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

12
kernel/cpu.c
View file

@ -2689,6 +2689,16 @@ 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;
@ -2699,7 +2709,7 @@ 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)