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

[Paul E. McKenney]

On Thu, Jul 30, 2026 at 05:08:13PM +0200, Jirka Hladky wrote:
> On Thu, Jul 30, 2026 at 4:53 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > 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.

That is more like I would expect.  Yes, there is added overhead, but it
should not be excessively expensive.  Though I suspect that arm64 could
also do better.  Easy for me to say, I know!  ;-)

							Thanx, Paul

> > In particular, I can see why the preempt_count() operations need to be
> > interrupt-safe, but I don't see why you would need barriers.  And
> > doesn't powerpc still use software interrupt disabling?  If so, why
> > not use that to simply software-disable interrupts around the
> > preempt_count() operations?
> >
> > What am I missing here?
> 
> That's a good question -- I don't know enough about the powerpc
> preempt_count implementation to answer this. Shrikanth, could you
> comment on whether removing the barriers or using software interrupt
> disabling around preempt_count is feasible?
> 
> Thank you
> Jirka
>