[PATCH v2 2/2] powerpc/smp: Disable steal from updating CPU capacity

[Srikar Dronamraju <srikar@linux.ibm.com>]

In a shared LPAR with SMT enabled, it has been observed that when a CPU
experiences steal time, it can trigger task migrations between sibling
CPUs. The idle CPU pulls a runnable task from its sibling that is
impacted by steal, making the previously busy CPU go idle. This reversal
can repeat continuously, resulting in ping-pong behavior between SMT
siblings.

To avoid migrations solely triggered by steal time, disable steal from
updating CPU capacity when running in shared processor mode.

lparstat
System Configuration
type=Shared mode=Uncapped smt=8 lcpu=72 mem=2139693696 kB cpus=64 ent=24.00

Noise case: (Ebizzy on 2 LPARs with similar configuration as above)
nr-ebizzy-threads  baseline  std-deviation  +patch    std-deviation
36                 1         (0.0345589)    1.01073   (0.0411082)
72                 1         (0.0387066)    1.12867   (0.029486)
96                 1         (0.013317)     1.05755   (0.0118292)
128                1         (0.028087)     1.04193   (0.027159)
144                1         (0.0103478)    1.07522   (0.0265476)
192                1         (0.0164666)    1.02177   (0.0164088)
256                1         (0.0241208)    0.977572  (0.0310648)
288                1         (0.0121516)    0.97529   (0.0263536)
384                1         (0.0128001)    0.967025  (0.0207603)
512                1         (0.0113173)    1.00975   (0.00753263)
576                1         (0.0126021)    1.01087   (0.0054196)
864                1         (0.0109194)    1.00369   (0.00987092)
1024               1         (0.0121474)    1.00338   (0.0122591)
1152               1         (0.013801)     1.0097    (0.0150391)

scaled perf stats for 72 thread case.
event         baseline  +patch
cycles        1         1.16993
instructions  1         1.14435
cs            1         0.913554
migrations    1         0.110884
faults        1         1.0005
cache-misses  1         1.68619

Observations:
- We see a drop in context-switches and migrations resulting in an
improvement in the records per second.

No-noise case: (Ebizzy on 1 LPARs with other LPAR being idle)
nr-ebizzy-threads  baseline  std-deviation  +patch    std-deviation
36                 1         (0.0451482)    1.01243   (0.0434088)
72                 1         (0.0308503)    1.06175   (0.0373877)
96                 1         (0.0500514)    1.13143   (0.0718754)
128                1         (0.0602872)    1.09909   (0.0375227)
144                1         (0.0843502)    1.07494   (0.0240824)
192                1         (0.0255402)    0.992734  (0.0615166)
256                1         (0.00653372)   0.982841  (0.00751558)
288                1         (0.00318369)   0.99093   (0.00960287)
384                1         (0.00272681)   0.974312  (0.0112133)
512                1         (0.00528486)   0.981207  (0.0125443)
576                1         (0.00491385)   0.992027  (0.0104948)
864                1         (0.0087057)    0.994927  (0.0143434)
1024               1         (0.010002)     0.992463  (0.00429322)
1152               1         (0.00720965)   1.00393   (0.012553)

Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
---
Changelog v1->v2:
v1: https://lkml.kernel.org/r/20251028104255.1892485-2-srikar@linux.ibm.com
Peter suggested to use static branch instead of sched feat

 arch/powerpc/kernel/smp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 5ac7084eebc0..0f7fae0b4420 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1694,8 +1694,11 @@ static void __init build_sched_topology(void)
 {
 	int i = 0;
 
-	if (is_shared_processor() && has_big_cores)
-		static_branch_enable(&splpar_asym_pack);
+	if (is_shared_processor()) {
+		if (has_big_cores)
+			static_branch_enable(&splpar_asym_pack);
+		sched_disable_steal_acct();
+	}
 
 #ifdef CONFIG_SCHED_SMT
 	if (has_big_cores) {
-- 
2.47.3