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

[Jirka Hladky]

On Mon, Jul 27, 2026 at 7:10 PM Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> wrote:
> That's full preemption mode.
>
> When preemption mode changes preempt_enable/disable which were just a
> barrier earlier now become real preemption points. If the code path
> repeatedly does the exact same thing, it might pop up.
> But, can we say is that number expected? it is difficult to put a
> number to it.

You were right -- my previous test conflated two variables. I've now
built a third kernel to separate them. All three are from the same
6.15-rc6 source, same machine (POWER10 lp11), no PREEMPT_DYNAMIC:

Config                                    Mode        PREEMPT_RCU
kill bogo-ops/sec
------------------------------------   ----------  -----------
-----------------
PREEMPT_VOLUNTARY=y   voluntary   no            105,014
PREEMPT_LAZY=y                lazy           no              86,878
PREEMPT=y                           full             yes             73,317

voluntary -> lazy                                  -17.3%
lazy -> full+PREEMPT_RCU               -15.6%
voluntary -> full+PREEMPT_RCU       -30.2%

The regression splits roughly 55/45 between the preemption mode
change and PREEMPT_RCU:

1) voluntary -> lazy (-17.3%): preempt_disable/enable becoming real
   preemption points, as you predicted. On ppc64le this is expensive
   because the kill() syscall path is very tight and hits these
   points heavily.

2) lazy -> full+PREEMPT_RCU (-15.6%): __rcu_read_lock/__rcu_read_unlock
   requiring lwsync/isync barriers on ppc64le.

> What I was asking is below. (You can do this only with below 7.0)
>
> Config A: CONFIG_PREEMPT_VOLUNTARY=y (CONFIG_PREEMPT_DYNAMIC=n)
> Config B: CONFIG_PREEMPT_VOLUNTARY=y (CONFIG_PREEMPT_DYNAMIC=y)

I couldn't do this exact test because 6.15-rc6 doesn't have
HAVE_PREEMPT_DYNAMIC_KEY for powerpc (that's your 6.16 patch), so
CONFIG_PREEMPT_DYNAMIC=y would be silently ignored. I would need a
6.16-6.19 kernel for this, which is before 7dadeaa6e851 removed
voluntary as an option.

But the PREEMPT_LAZY test above achieves the same goal: it isolates
the preemption mode cost without PREEMPT_RCU.

> Are you saying you see regression with voluntary with
> CONFIG_PREEMPT_DYNAMIC=y?

Yes. On the ELN kernels with CONFIG_PREEMPT_DYNAMIC=y, the regression
appears at 6.16 when HAVE_PREEMPT_DYNAMIC_KEY is added, because:
1) PREEMPT_DYNAMIC forces the runtime mode to lazy/full (voluntary
   is no longer available on architectures with ARCH_HAS_PREEMPT_LAZY)
2) PREEMPT_DYNAMIC pulls in PREEMPT_RCU

Both contribute to the ~30% total regression.

Jirka