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

[Vincent Guittot]

Hi Srikar,

On Wed, 29 Oct 2025 at 07:09, Srikar Dronamraju <srikar at linux.ibm.com> wrote:
>
> 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

IIUC, the migration is triggered by the reduced capacity case when
there is 1 task on the CPU

> can repeat continuously, resulting in ping-pong behavior between SMT
> siblings.

Does it mean that the vCPU generates its own steal time or is it
because other vcpus are already running on the other CPU and they
starts to steal time on the sibling vCPU

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

You are disabling the steal time accounting only for your arch. Does
it mean that only powerpc are impacted by this effect ?

>
> 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 at 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
>