Re: [PATCH v5 0/8] Generic IRQ entry/exit support for powerpc

Hi Mukesh, Shrikanth, Madhavan,

I'm seeing a ~19% throughput regression on ppc64le (POWER10) in
syscall-heavy workloads between kernel 7.1 and 7.2-rc4, which I've
narrowed down to the GENERIC_ENTRY enablement for powerpc (commit
bee25f97ad24 "powerpc: Enable GENERIC_ENTRY feature").

The regression manifests specifically when SELinux is enabled. Regression
disappears with selinux=0. With SELinux disabled, 7.2 is actually
slightly *faster* than 7.1.

Test: stress-ng --kill 1 -t 23 (single-thread kill() syscall loop)
Machine: IBM,9080-HEX POWER10, 8 vCPUs (1 core SMT-8), 64 GiB

Results (bogo-ops/sec, higher is better):

Kernel   Enforcing   Permissive   Disabled    SELinux tax
------   ---------   ----------   --------    -----------
7.1      69,107      70,248       93,566      -26.1%
7.2-rc4  56,247      59,075       99,240      -43.3%
Delta    -18.6%      -15.9%       +6.1%

Key observations:
- SELinux permissive ~ enforcing on both kernels (overhead is in the
  code path, not policy evaluation)
- SELinux disabled: 7.2 is 6% faster than 7.1 -- the generic entry
  path itself is not slower for the base syscall
- The SELinux "tax" nearly doubles from 26% to 43% between 7.1->7.2
- No changes to security/selinux/avc.c between v7.1 and v7.2-rc4

The kill() hot path (sys_kill -> check_kill_permission -> security_task_kill
-> selinux_task_kill -> avc_has_perm -> avc_lookup) wraps every AVC lookup
in rcu_read_lock()/rcu_read_unlock(). On ppc64le with PREEMPT_RCU, each
pair requires lwsync/isync barriers. I suspect the generic entry path
changes something in how context tracking, tracing, or RCU interacts
with the syscall fast path that amplifies the per-call RCU cost.

Note: this is on top of a separate ~36% regression from 6.12->7.1 caused
by HAVE_PREEMPT_DYNAMIC_KEY enabling CONFIG_PREEMPT_RCU on ppc64le
(commit 6ad7751537e8), which I reported separately on the v4 thread.

Thank you
Jirka

On Mon, Apr 27, 2026 at 2:33 PM Mukesh Kumar Chaurasiya (IBM)
<[email protected]> wrote:
>
> Adding support for the generic irq entry/exit handling for PowerPC. The
> goal is to bring PowerPC in line with other architectures that already
> use the common irq entry infrastructure, reducing duplicated code and
> making it easier to share future changes in entry/exit paths.
>
> This is slightly tested of ppc64le and ppc32.
>
> The performance benchmarks are below:
>
> perf bench syscall usec/op (-ve is improvement)
>
> | Syscall | Base        | test        | change % |
> | ------- | ----------- | ----------- | -------- |
> | basic   | 0.093543    | 0.093023    | -0.56    |
> | execve  | 446.557781  | 450.107172  | +0.79    |
> | fork    | 1142.204391 | 1156.377214 | +1.24    |
> | getpgid | 0.097666    | 0.092677    | -5.11    |
>
> perf bench syscall ops/sec (+ve is improvement)
>
> | Syscall | Base     | New      | change % |
> | ------- | -------- | -------- | -------- |
> | basic   | 10690548 | 10750140 | +0.56    |
> | execve  | 2239     | 2221     | -0.80    |
> | fork    | 875      | 864      | -1.26    |
> | getpgid | 10239026 | 10790324 | +5.38    |
>
>
> IPI latency benchmark (-ve is improvement)
>
> | Metric         | Base (ns)     | New (ns)      | % Change |
> | -------------- | ------------- | ------------- | -------- |
> | Dry run        | 583136.56     | 584136.35     | 0.17%    |
> | Self IPI       | 4167393.42    | 4149093.90    | -0.44%   |
> | Normal IPI     | 61769347.82   | 61753728.39   | -0.03%   |
> | Broadcast IPI  | 2235584825.02 | 2227521401.45 | -0.36%   |
> | Broadcast lock | 2164964433.31 | 2125658641.76 | -1.82%   |
>
>
> Thats very close to performance earlier with arch specific handling.
>
> Tests done:
>  - Build and boot on ppc64le pseries.
>  - Build and boot on ppc64le powernv8 powernv9 powernv10.
>  - Build and boot on ppc32.
>  - Performance benchmark done with perf syscall basic on pseries.
>
> Changelog:
> V4 -> V5:
>  - Rebased on latest mainline
> V4: https://lore.kernel.org/all/[email protected]/
>
> V3 -> V4:
>  - Fixed the issue in older gcc version where linker couldn't find
>    mem functions
>  - Merged IRQ enable and syscall enable into a single patch
>  - Cleanup for unused functions done in separate patch.
>  - Some other cosmetic changes
> V3: https://lore.kernel.org/all/[email protected]/
>
> V2 -> V3:
>  - #ifdef CONFIG_GENERIC_IRQ_ENTRY removed from unnecessary places
>  - Some functions made __always_inline
>  - pt_regs padding changed to match 16byte interrupt stack alignment
>  - And some cosmetic changes from reviews from earlier patch
> V2: https://lore.kernel.org/all/[email protected]/
>
> V1 -> V2:
>  - Fix an issue where context tracking was showing warnings for
>    incorrect context
> V1: https://lore.kernel.org/all/[email protected]/
>
> RFC -> PATCH V1:
>  - Fix for ppc32 spitting out kuap lock warnings.
>  - ppc64le powernv8 crash fix.
>  - Review comments incorporated from previous RFC.
> RFC https://lore.kernel.org/all/[email protected]/
>
> Mukesh Kumar Chaurasiya (8):
>   powerpc: rename arch_irq_disabled_regs
>   powerpc: Prepare to build with generic entry/exit framework
>   powerpc: introduce arch_enter_from_user_mode
>   powerpc: Introduce syscall exit arch functions
>   powerpc: add exit_flags field in pt_regs
>   powerpc: Prepare for IRQ entry exit
>   powerpc: Enable GENERIC_ENTRY feature
>   powerpc: Remove unused functions
>
>  arch/powerpc/Kconfig                    |   1 +
>  arch/powerpc/include/asm/entry-common.h | 533 ++++++++++++++++++++++++
>  arch/powerpc/include/asm/hw_irq.h       |   4 +-
>  arch/powerpc/include/asm/interrupt.h    | 386 +++--------------
>  arch/powerpc/include/asm/kasan.h        |  15 +-
>  arch/powerpc/include/asm/ptrace.h       |   6 +-
>  arch/powerpc/include/asm/signal.h       |   1 -
>  arch/powerpc/include/asm/stacktrace.h   |   6 +
>  arch/powerpc/include/asm/syscall.h      |   5 +
>  arch/powerpc/include/asm/thread_info.h  |   1 +
>  arch/powerpc/include/uapi/asm/ptrace.h  |  14 +-
>  arch/powerpc/kernel/interrupt.c         | 254 ++---------
>  arch/powerpc/kernel/ptrace/ptrace.c     | 142 +------
>  arch/powerpc/kernel/signal.c            |  25 +-
>  arch/powerpc/kernel/syscall.c           | 119 +-----
>  arch/powerpc/kernel/traps.c             |   2 +-
>  arch/powerpc/kernel/watchdog.c          |   2 +-
>  arch/powerpc/perf/core-book3s.c         |   2 +-
>  18 files changed, 690 insertions(+), 828 deletions(-)
>  create mode 100644 arch/powerpc/include/asm/entry-common.h
>
> --
> 2.53.0
>
>


-- 
-Jirka