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

[Paul E. McKenney]

On Fri, Jul 31, 2026 at 11:27:06PM +0530, Shrikanth Hegde wrote:
> Hi Jirka.
> 
> On 7/31/26 7:20 PM, Jirka Hladky wrote:
> > On Fri, Jul 31, 2026 at 10:03 AM Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> 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.

For some reason, I remembered powerpc software-disabling interrupts,
but when I go look now, there is no such thing.  Maybe powerpc used to
but no longer does.  Or maybe I am just hallucinating.  ;-)

							Thanx, Paul