Re: [PATCH v4 1/1] powerpc: enable dynamic preemption

[Shrikanth Hegde <sshegde@linux.ibm.com>]

+cc paul for any further/RCU insights.

On 7/27/26 12:03 AM, Jirka Hladky wrote:
>    Hi Shrikanth, Christophe,
> 

Hi Jirka, thanks for the report.

>   I'm seeing a significant performance regression on ppc64le after this
>   patch landed in 6.16, caused by CONFIG_PREEMPT_RCU becoming active
>   once HAVE_PREEMPT_DYNAMIC_KEY is selected.
> 

This is true only if user selected PREEMPT_DYNAMIC option i think.

config PREEMPT_RCU
         bool
         default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
         select TREE_RCU


>   Benchmark: stress-ng kill stressor (tight kill() syscall loop),
>   single thread, POWER10 LPAR (8 vCPUs, 1 core SMT-8).
> 
>   Bisected across Fedora ELN kernel builds on ppc64le:
> 
>     kernel               CONFIG_PREEMPT_RCU   kill bogo-ops/sec
>   ---  6.15-rc6 (eln148)    no                   103,207
>     6.16     (eln150)          yes                   70,281    (-32%)
>     6.18     (eln154)          yes                   72,552    (-30%)
> 

Does your preemption mode remain the same in two cases?

>   For comparison, x86_64 (AMD EPYC 9355P) with the same config change
>   shows only a 2.8% regression:
> 
>     6.12 x86_64          37,436
>     7.2  x86_64           36,392    (-2.8%)
> 
>   perf report shows the overhead comes from rcu_read_lock/unlock in the
>   SELinux AVC path (check_kill_permission -> security_task_kill ->
>   selinux_task_kill -> avc_has_perm -> avc_lookup):
> 
>     Function               6.15 (no PREEMPT_RCU)   6.16 (PREEMPT_RCU)
>   ---  avc_lookup                          15.23%              24.79%
>     __rcu_read_lock                      ~0%                 4.52%
>     __rcu_read_unlock                    ~0%                 4.17%
>     selinux_task_kill                    6.23%               7.35%
>     audit_signal_info*                   0.94%               3.59%
> 
>   On x86_64, rcu_read_lock/unlock are cheap thanks to static calls
>   (HAVE_PREEMPT_DYNAMIC_CALL). On ppc64le with the KEY-based
>   implementation, the weak memory model requires real barriers
>   (lwsync/isync) making each RCU read-side critical section
>   significantly more expensive.

Weak memory model would need barriers irrespective of HAVE_PREEMPT_DYNAMIC_CALL
or HAVE_PREEMPT_DYNAMIC_KEY. That's my assumption. I will look
more into it. Also i don't know much about PREEMPT_RCU. So might take a while.

> 
>   This aligns with Christophe's earlier review comment that
>   HAVE_PREEMPT_DYNAMIC_CALL should be more performant. Would
>   implementing static calls for ppc64 be feasible to close this gap?
> 

Static key too is expected to minimal cost. There maybe more into this.

>   Test details:
>   - Machine: IBM POWER10 (pvr 0080 0200), pHyp virtualization
>   - stress-ng 0.21.03, gcc 14.3.1, glibc 2.39
>   - Tuned profile: virtual-guest
>   - SELinux: enforcing (permissive recovers only ~7%)
> 
>   Happy to run additional tests if needed.
> 
>