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

[Paul E. McKenney]

On Mon, Jul 27, 2026 at 09:48:25AM +0530, Shrikanth Hegde wrote:
> +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

Yes, if you select PREEMPT_DYNAMIC, RCU doesn't have much choice but to
choose PREEMPT_RCU.  Something about not having the guts to implement an
RCU that dynamically switches between its preemptible and non-preemptible
variants.  Cowardly of me, I know!

So if you want non-preemptible RCU, you need to turn off PREEMPT_DYNAMIC.

> >   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%

Yes, non-preemptible RCU's __rcu_read_{,un}lock() are (almost) no-ops,
but preemptible RCU must actually execute real code.  But I would not
expect *this* much overhead.

> >     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.

OK, if you are executing an isync or an lwsync instruction in each call
to __rcu_read_{,un}lock(), that would explain the overhead.

CONFIG_PREEMPT_DYNAMIC=n for the win?

							Thanx, Paul

> >   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.
> > 
> >