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

Hi Jirka.

On 7/31/26 7:20 PM, Jirka Hladky wrote:

On Fri, Jul 31, 2026 at 10:03 AM Shrikanth Hegde [off-list ref] wrote:

quoted

Coming into numbers, I did notice later, arm64 numbers are almost 20x less.
But if we compare the time taken for each operations, it tells a
different story.

The bottom line: PREEMPT_DYNAMIC/preempt_count overhead is present
on both RISC architectures but its visibility depends heavily on the
workload profile and hardware topology.

quoted

PS: I tried porting some of those changes in arm64 to powerpc to
split preempt count into two. but i don;t see much improvement
there.

I tried moving to PACA plus splitting 64 bit into need_resched and preempt
count. But it doesn;t move the needle. But it maybe worth pursuing,
since it could help preemption modes. It needs more work and needs numbers
across benchmark.

Thanks for trying. Paul questioned whether barriers are actually
needed for preempt_count, or whether software interrupt disabling
would suffice (his words: "I can see why the preempt_count()
operations need to be interrupt-safe, but I don't see why you would
need barriers"). Any thoughts on whether that's feasible for
powerpc?

Barrier are likely not the concern as it is mostly nop.
Also, soft disabling interrupts needs to arch specific hooks to setup
before preempt_disable and replay interrupts after preempt_enable.
I don't think it would benefit since interrupt disabling during this likely
shouldn't happen. Maybe I don't understand how that could work.