linux-kernel - [PATCH v2 1/2] cpu/SMT: Enable SMT only if a core is online

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240731030126.956210-2-nysal@linux.ibm.com>
Date: Wed, 31 Jul 2024 08:31:12 +0530
From: "Nysal Jan K.A." <nysal@...ux.ibm.com>
To: Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>
Cc: Tyrel Datwyler <tyreld@...ux.ibm.com>, Michal Suchanek <msuchanek@...e.de>,
        "Nysal Jan K.A" <nysal@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Eric DeVolder <eric.devolder@...cle.com>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Laurent Dufour <ldufour@...ux.ibm.com>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2 1/2] cpu/SMT: Enable SMT only if a core is online

From: "Nysal Jan K.A" <nysal@...ux.ibm.com>

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@...ux.ibm.com>
Closes: https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
Signed-off-by: Nysal Jan K.A <nysal@...ux.ibm.com>
---
 Documentation/ABI/testing/sysfs-devices-system-cpu |  3 ++-
 kernel/cpu.c                                       | 12 +++++++++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 325873385b71..de725ca3be82 100644
--- 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 1209ddaec026..b1fd2a3db91a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -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)
-- 
2.35.3