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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Jirka.

On 7/31/26 7:20 PM, Jirka Hladky wrote:
> On Fri, Jul 31, 2026 at 10:03 AM Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
>> 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.
> 
>> 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.