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

On Mon, Jul 27, 2026 at 6:16 PM Paul E. McKenney [off-list ref] wrote:

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.

I've now isolated the PREEMPT_RCU cost with a controlled experiment.
Built two kernels from the same 6.15-rc6 upstream source on the same
POWER10 machine, no CONFIG_PREEMPT_DYNAMIC in either case:

Config A: CONFIG_PREEMPT_VOLUNTARY=y (no PREEMPT_RCU)
Config B: CONFIG_PREEMPT=y           (PREEMPT_RCU=y)

Results (stress-ng --kill 1 -t 23, SELinux enforcing):

Kernel                       PREEMPT_RCU   kill bogo-ops/sec
---------------------------  -----------   -----------------
6.15-rc6-voluntary-test      no            105,014
6.15-rc6-preempt-test        yes            73,317
Delta                                      -30.2%

PREEMPT_RCU alone accounts for ~30% on this workload. The kill()
path hits rcu_read_lock/unlock very heavily through the SELinux AVC
(avc_has_perm -> avc_lookup wraps every hash table lookup in an RCU
read-side critical section).

This also answers Shrikanth's question about whether the regression
is from the preemption mode change (voluntary -> lazy) or from
PREEMPT_RCU. Since no PREEMPT_DYNAMIC is involved in either build,
the preemption mode is not a factor. Additionally, switching between
full and lazy at runtime on 7.1 showed only ~1% difference (57,476
vs 56,892), further confirming the mode doesn't matter.

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

Yes, that's what perf shows. __rcu_read_lock and __rcu_read_unlock
together consume ~9% of total cycles on ppc64le with PREEMPT_RCU,
vs essentially 0% without.

CONFIG_PREEMPT_DYNAMIC=n for the win?

That's the simplest distro workaround for ppc64le. But since commit
7dadeaa6e851 ("sched: Further restrict the preemption modes")
removed PREEMPT_VOLUNTARY as an option for architectures with
ARCH_HAS_PREEMPT_LAZY (which includes powerpc), distros that want
voluntary preemption on ppc64le would need to disable PREEMPT_DYNAMIC
anyway.

Is there any path to reducing the barrier cost in
__rcu_read_lock/__rcu_read_unlock on weakly-ordered architectures?
Or is the current implementation fundamentally constrained by the
memory model?

Jirka