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

[Jirka Hladky]

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.

Good catch. I investigated why the arm64 baseline is so much slower.
The 20x gap is due to cross-process signal delivery cost on the
128-core Ampere Altra Max, not preempt_count.

Self-kill (same process) vs cross-process kill on arm64:

  self-kill (kill to self):      1.16 us/op   (863k ops/sec)
  cross-process kill:           54.06 us/op   (18.5k ops/sec)

The stress-ng kill stressor sends signals between parent and child
processes, so it's dominated by the cross-process path (IPI, wakeup,
context switch). On the 128-core Ampere, this is expensive.

In contrast, the ppc64le test ran on 8 vCPUs sharing 1 physical core
(SMT-8 LPAR), where cross-thread signaling is much cheaper.

For reference, basic syscall latency (getppid) is only 1.6x different:

  ppc64le POWER10:  0.106 us/op
  aarch64 Ampere:   0.171 us/op

So the architectures are comparable for simple syscalls. The kill
stressor gap is about signal delivery topology, not CPU speed.

I also reran the arm64 tests with "taskset -c 0-7" to match the
ppc64le vCPU count. Results barely changed (kill uses only 2-3
threads), confirming this is not a core count issue but a cross-core
signaling cost:

With taskset -c 0-7 on arm64 (Ampere Altra Max, 6.18):

Kernel                  PREEMPT_RCU   kill ops/sec   us/op
----------------------  -----------   ------------   ------
6.18.0-vol-nodynamic    no            6,171          162.0
6.18.0-vol-dynamic      yes           5,477          182.6
Delta                                 -11.2%         +20.6 us/op

Updated per-operation comparison:

            PREEMPT_DYNAMIC
ppc64le         off              9.19 us/op
ppc64le         on              14.66 us/op
delta                           +5.47 us/op

aarch64         off            162.0  us/op
aarch64         on             182.6  us/op
delta                          +20.6  us/op

The absolute delta is larger on arm64 (+20.6 us vs +5.47 us), but
as a percentage of total cost, ppc64le is hit harder (-37% vs -11%)
because its baseline is dominated by the syscall/security path where
preempt_count operations concentrate, whereas arm64's baseline is
dominated by signal delivery overhead that masks the preempt_count
cost.

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.

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?

Jirka