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

On Fri, Jul 31, 2026 at 7:03 AM Christophe Leroy (CS GROUP)
[off-list ref] wrote:

Le 30/07/2026 à 19:10, Shrikanth Hegde a écrit :

quoted

Hi Jirka, Paul,


+cc will

On 7/30/26 8:38 PM, Jirka Hladky wrote:

quoted

On Thu, Jul 30, 2026 at 4:53 PM Paul E. McKenney [off-list ref]
wrote:

quoted

But is this really a fundamental RISC cost?  For example, does arm64
see the same performance issues?

We tested arm64 (Ampere Altra Max) with the same controlled
experiment -- two 6.18 kernels, both voluntary, differing only in
PREEMPT_DYNAMIC:

Arch     PREEMPT_DYNAMIC   kill bogo-ops/sec   Delta
-------  ---------------   -----------------   -----
ppc64le  off               108,836
ppc64le  on                 68,197             -37.3%
aarch64  off                 5,538
aarch64  on                  5,082              -8.2%

arm64 sees -8.2% vs ppc64le's -37.3%. So arm64 is affected but
much less severely.

Ouch!. But that's good to know.

Might be a stupid question, but what is your .config ?
Are you sure it doesn't contain CONFIG_DEBUG_PREEMPT ?

I have double-checked that and it's not set:
# CONFIG_DEBUG_PREEMPT is not set

Thanks
Jirka