Subject: [PATCH 2/2] powerpc/smp: Disable ACCT_STEAL for shared LPARs

From: Srikar Dronamraju <[email-masked]>
Date: Tue, 28 Oct 2025 16:12:55 +0530

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 the
ACCT_STEAL scheduling feature 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)    1.02358   (0.0346247)
72                 1         (0.0387066)    1.11729   (0.0215052)
96                 1         (0.013317)     1.07751   (0.014656)
128                1         (0.028087)     1.0585    (0.0173575)
144                1         (0.0103478)    1.11785   (0.0472121)
192                1         (0.0164666)    1.0212    (0.0226717)
256                1         (0.0241208)    0.969056  (0.0169747)
288                1         (0.0121516)    0.971862  (0.0190453)

scaled perf stats for 72 thread case.
event         baseline  +patch
cycles        1         1.16475
instructions  1         1.13198
cs            1         0.914774
migrations    1         0.116058
faults        1         0.94104
cache-misses  1         1.75184

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)    0.985758  (0.0204456)
72                 1         (0.0308503)    1.0288    (0.065893)
96                 1         (0.0500514)    1.07178   (0.0376889)
128                1         (0.0602872)    0.986705  (0.0467856)
144                1         (0.0843502)    1.04157   (0.0626338)
192                1         (0.0255402)    1.03327   (0.0975257)
256                1         (0.00653372)   1.04572   (0.00576901)
288                1         (0.00318369)   1.04578   (0.0115398)

Signed-off-by: Srikar Dronamraju <[email-masked]>
---
 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..d80053f0a05e 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);
+		steal_updates_cpu_capacity(false);
+	}
 
 #ifdef CONFIG_SCHED_SMT
 	if (has_big_cores) {
-- 
2.47.3