[PATCH v10 0/9] riscv: optimize percpu access

[Yunhui Cui]

RISC-V has no asm/percpu.h, so this_cpu_*() read-modify-write
operations use generic fallbacks that disable local interrupts. Per-CPU
addressing also derives the current offset from the CPU number and
__per_cpu_offset[].

Cache the per-CPU offset in thread_info and implement preemptible this_cpu
operations using recoverable GPR critical sections. Each operation records
the registers used to form its per-CPU address so exception return can
repair them if the task resumes on a different CPU. Use AMOs for
native-width operations. Use Zabha for 8-bit and 16-bit operations when
available, or a 32-bit LR/SC fallback otherwise. Finally, select
HAVE_CMPXCHG_LOCAL so common per-CPU counter users can use
this_cpu_try_cmpxchg().

The recoverable-GPR scheme follows the arm64 design. Only the protocol is
common, however: RISC-V uses a cached offset in thread_info, its own GPR
encoding and exception entry code, and AMO/LR/SC operation sequences.
Factoring these pieces into generic code would share little beyond the
field layout while coupling architecture entry paths, so keep the
implementation under arch/riscv.

As with arm64's SDEI path, independent NMI-like entry paths must
participate in the entry-state protocol directly. Keep the save, clear and
restore operations in common RISC-V assembly macros so SBI Supervisor
Software Events (SSE) can preserve an interrupted per-CPU operation. SBI
SSE support is developed separately and is not a dependency of this
series.

With CONFIG_PREEMPT_LAZY=y, GCC 15, the same linux-next base and
configuration, an in-kernel microbenchmark pinned to one CPU ran each
operation 2,000,000 times for 15 rounds. The table reports the median
empty-loop-adjusted time; lower is better.

  operation                 v9 (ns/op)   v10 (ns/op)   change
  this_cpu_read(u64)            5.668         0.333     -94.1%
  this_cpu_write(u64)           5.668         0.395     -93.0%
  this_cpu_add(u64)             8.002         8.004      +0.0%
  this_cpu_add_return(u64)      8.335         8.004      -4.0%
  this_cpu_xchg(u64)            8.278         8.004      -3.3%
  this_cpu_cmpxchg(u64)        18.713        16.133     -13.8%

With the same configuration, vmlinux .text decreased from 14,369,618 to
14,212,882 bytes (-156,736 bytes, -1.09%). percpu_counter_add_batch()
decreased from 330 to 228 bytes.

Changes since v9:

- Rework the this_cpu operations to remain preemptible using recoverable
  GPR critical sections, as suggested by David Laight and Jisheng Zhang.
- Put the cached per-CPU offset first, then split the exception/fixup
  infrastructure and operation classes into independently reviewable
  patches.
- Keep GPR-state save and restore in reusable assembly macros for NMI-like
  entry paths such as SBI SSE.
- Keep masked LR/SC cmpxchg operands XLEN-wide so RV64 high-byte and
  high-halfword comparisons are not affected by LR.W sign extension.
- Force both halves of the AMOCAS.Q new value into fixed registers so the
  implicit register pair remains valid when either half is zero.
- Use TYPEOF_UNQUAL() in wrappers and fully parenthesize this_cpu_write()
  values.

Changes since v8:

- Drop the PIO helper patch, which has already been merged.
- Rename the first patch and update its changelog as suggested by
  Davidlohr Bueso.
- Use value-returning AMOs instead of rd=x0 for CPU-local operations.
- Remove unnecessary memory clobbers from the new relaxed AMO and LR/SC
  code.
- Use .option arch,+zabha for the Zabha inline asm, matching the
  existing RISC-V cmpxchg implementation.
- Isolate arch_cmpxchg_relaxed() behind typed always-inline helpers to
  avoid macro variable capture while preserving atomic instrumentation.
- Make the cmpxchg128 fallback use the already resolved per-CPU pointer,
  avoiding the same macro variable capture.
- Copy the cached per-CPU offset from prev to next during context switch,
  as suggested by Thomas Gleixner.
- Initialize secondary idle tasks in the common startup path, covering
  parallel bringup and CPU hotplug, and remove the duplicate initialization
  from smp_callin(), as reported by Sashiko.
- Select HAVE_CMPXCHG_LOCAL in a separate patch.

Yunhui Cui (9):
  riscv: store percpu offset into thread_info
  riscv: add infrastructure for preemptible this_cpu operations
  riscv: implement preemptible this_cpu read and write operations
  riscv: implement preemptible this_cpu void RMW operations
  riscv: implement preemptible this_cpu add_return operations
  riscv: implement preemptible this_cpu xchg operations
  riscv: implement preemptible this_cpu cmpxchg operations
  riscv: implement preemptible this_cpu cmpxchg128
  riscv: select HAVE_CMPXCHG_LOCAL

 arch/riscv/Kconfig                   |   1 +
 arch/riscv/include/asm/asm.h         |  25 +-
 arch/riscv/include/asm/percpu.h      | 701 +++++++++++++++++++++++++++
 arch/riscv/include/asm/ptrace.h      |  15 +
 arch/riscv/include/asm/switch_to.h   |   9 +
 arch/riscv/include/asm/thread_info.h |   4 +-
 arch/riscv/kernel/Makefile           |   1 +
 arch/riscv/kernel/asm-offsets.c      |   4 +-
 arch/riscv/kernel/entry-common.c     |  32 ++
 arch/riscv/kernel/entry.S            |  10 +
 arch/riscv/kernel/smpboot.c          |  10 +-
 arch/riscv/net/bpf_jit_comp64.c      |   9 +-
 12 files changed, 805 insertions(+), 16 deletions(-)
 create mode 100644 arch/riscv/include/asm/percpu.h
 create mode 100644 arch/riscv/kernel/entry-common.c


base-commit: ddfe51b0a0f5dc9e415cc2c8ad8db17532dd426b
-- 
2.39.5