[PATCH 2/2] powerpc/pseries/cpuhp: respect current SMT when adding new CPU

[Laurent Dufour]

When a new CPU is added, the kernel is activating all its threads. This
leads to weird, but functional, result when adding CPU on a SMT 4 system
for instance.

Here the newly added CPU 1 has 8 threads while the other one has 4 threads
active (system has been booted with the 'smt-enabled=4' kernel option):

ltcden3-lp12:~ # ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7
Core   1:    8*    9*   10*   11*   12*   13*   14*   15*

We rely on the newly pseries_smt value which should be updated when
changing the SMT level by ppc64_cpu --smt=x and at boot time using the
smt-enabled kernel option.

This way on a LPAR running in SMT=4, newly added CPU will be running 4
threads, which is what a end user would expect.

Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
---
 arch/powerpc/platforms/pseries/hotplug-cpu.c | 18 +++++++++++++-----
 arch/powerpc/platforms/pseries/smp.c         |  2 +-
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 1a3cb313976a..e623ed8649b3 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -382,7 +382,7 @@ static int dlpar_online_cpu(struct device_node *dn)
 {
 	int rc = 0;
 	unsigned int cpu;
-	int len, nthreads, i;
+	int len, nthreads, i, smt;
 	const __be32 *intserv;
 	u32 thread;
 
@@ -392,6 +392,11 @@ static int dlpar_online_cpu(struct device_node *dn)
 
 	nthreads = len / sizeof(u32);
 
+	smt = READ_ONCE(pseries_smt);
+	/* We should online at least one thread */
+	if (smt < 1)
+		smt = 1;
+
 	cpu_maps_update_begin();
 	for (i = 0; i < nthreads; i++) {
 		thread = be32_to_cpu(intserv[i]);
@@ -400,10 +405,13 @@ static int dlpar_online_cpu(struct device_node *dn)
 				continue;
 			cpu_maps_update_done();
 			find_and_update_cpu_nid(cpu);
-			rc = device_online(get_cpu_device(cpu));
-			if (rc) {
-				dlpar_offline_cpu(dn);
-				goto out;
+			/* Don't active CPU over the current SMT setting */
+			if (smt-- > 0) {
+				rc = device_online(get_cpu_device(cpu));
+				if (rc) {
+					dlpar_offline_cpu(dn);
+					goto out;
+				}
 			}
 			cpu_maps_update_begin();
 
diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index 6c382922f8f3..ef8070651846 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -295,7 +295,7 @@ static ssize_t pseries_smt_store(struct class *class,
 		return -EINVAL;
 	}
 
-	pseries_smt = smt;
+	WRITE_ONCE(pseries_smt, smt);
 
 	return count;
 }
-- 
2.40.0