On Thu, Aug 15, 2024 at 03:27:31PM GMT, Alexandre Ghiti wrote:

Hi Andrew,

On 01/08/2024 08:53, Alexandre Ghiti wrote:

quoted

On Wed, Jul 31, 2024 at 5:29 PM Andrew Jones [off-list ref] wrote:

quoted

On Wed, Jul 31, 2024 at 09:24:05AM GMT, Alexandre Ghiti wrote:

quoted

In order to produce a generic kernel, a user can select
CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
spinlock implementation if Zabha or Ziccrse are not present.

Note that we can't use alternatives here because the discovery of
extensions is done too late and we need to start with the qspinlock
implementation because the ticket spinlock implementation would pollute
the spinlock value, so let's use static keys.

This is largely based on Guo's work and Leonardo reviews at [1].

Link: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-guoren@kernel.org/ (local) [1]
Signed-off-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Alexandre Ghiti <redacted>
---
  .../locking/queued-spinlocks/arch-support.txt |  2 +-
  arch/riscv/Kconfig                            | 29 +++++++++++++
  arch/riscv/include/asm/Kbuild                 |  4 +-
  arch/riscv/include/asm/spinlock.h             | 43 +++++++++++++++++++
  arch/riscv/kernel/setup.c                     | 38 ++++++++++++++++
  include/asm-generic/qspinlock.h               |  2 +
  include/asm-generic/ticket_spinlock.h         |  2 +
  7 files changed, 118 insertions(+), 2 deletions(-)
  create mode 100644 arch/riscv/include/asm/spinlock.h

diff --git a/Documentation/features/locking/queued-spinlocks/arch-support.txt b/Documentation/features/locking/queued-spinlocks/arch-support.txt
index 22f2990392ff..cf26042480e2 100644
--- a/Documentation/features/locking/queued-spinlocks/arch-support.txt
+++ b/Documentation/features/locking/queued-spinlocks/arch-support.txt

@@ -20,7 +20,7 @@
      |    openrisc: |  ok  |
      |      parisc: | TODO |
      |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
      |        s390: | TODO |
      |          sh: | TODO |
      |       sparc: |  ok  |

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index ef55ab94027e..c9ff8081efc1 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig

@@ -79,6 +79,7 @@ config RISCV
       select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
       select ARCH_WANTS_NO_INSTR
       select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
+     select ARCH_WEAK_RELEASE_ACQUIRE if ARCH_USE_QUEUED_SPINLOCKS

Why do we need this? Also, we presumably would prefer not to have it
when we end up using ticket spinlocks when combo spinlocks is selected.
Is there no way to avoid it?

I'll let Andrea answer this as he asked for it.

quoted

quoted

       select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
       select BUILDTIME_TABLE_SORT if MMU
       select CLINT_TIMER if RISCV_M_MODE

@@ -488,6 +489,34 @@ config NODES_SHIFT
         Specify the maximum number of NUMA Nodes available on the target
         system.  Increases memory reserved to accommodate various tables.

+choice
+     prompt "RISC-V spinlock type"
+     default RISCV_COMBO_SPINLOCKS
+
+config RISCV_TICKET_SPINLOCKS
+     bool "Using ticket spinlock"
+
+config RISCV_QUEUED_SPINLOCKS
+     bool "Using queued spinlock"
+     depends on SMP && MMU && NONPORTABLE
+     select ARCH_USE_QUEUED_SPINLOCKS
+     help
+       The queued spinlock implementation requires the forward progress
+       guarantee of cmpxchg()/xchg() atomic operations: CAS with Zabha or
+       LR/SC with Ziccrse provide such guarantee.
+
+       Select this if and only if Zabha or Ziccrse is available on your
+       platform.

Maybe some text recommending combo spinlocks here? As it stands it sounds
like enabling queued spinlocks is a bad idea for anybody that doesn't know
what platforms will run the kernel they're building, which is all distros.

That's NONPORTABLE, so people enabling this config are supposed to
know that right?

quoted

quoted

+
+config RISCV_COMBO_SPINLOCKS
+     bool "Using combo spinlock"
+     depends on SMP && MMU
+     select ARCH_USE_QUEUED_SPINLOCKS
+     help
+       Embed both queued spinlock and ticket lock so that the spinlock
+       implementation can be chosen at runtime.

nit: Add a blank line here

Done

quoted

quoted

+endchoice
+
  config RISCV_ALTERNATIVE
       bool
       depends on !XIP_KERNEL

diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 5c589770f2a8..1c2618c964f0 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild

@@ -5,10 +5,12 @@ syscall-y += syscall_table_64.h
  generic-y += early_ioremap.h
  generic-y += flat.h
  generic-y += kvm_para.h
+generic-y += mcs_spinlock.h
  generic-y += parport.h
-generic-y += spinlock.h
  generic-y += spinlock_types.h
+generic-y += ticket_spinlock.h
  generic-y += qrwlock.h
  generic-y += qrwlock_types.h
+generic-y += qspinlock.h
  generic-y += user.h
  generic-y += vmlinux.lds.h

diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
new file mode 100644
index 000000000000..503aef31db83
--- /dev/null
+++ b/arch/riscv/include/asm/spinlock.h

@@ -0,0 +1,43 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __ASM_RISCV_SPINLOCK_H
+#define __ASM_RISCV_SPINLOCK_H
+
+#ifdef CONFIG_RISCV_COMBO_SPINLOCKS
+#define _Q_PENDING_LOOPS     (1 << 9)
+
+#define __no_arch_spinlock_redefine
+#include <asm/ticket_spinlock.h>
+#include <asm/qspinlock.h>
+#include <asm/alternative.h>

We need asm/jump_label.h instead of asm/alternative.h, but...

quoted

+
+DECLARE_STATIC_KEY_TRUE(qspinlock_key);
+
+#define SPINLOCK_BASE_DECLARE(op, type, type_lock)                   \
+static __always_inline type arch_spin_##op(type_lock lock)           \
+{                                                                    \
+     if (static_branch_unlikely(&qspinlock_key))                     \
+             return queued_spin_##op(lock);                          \
+     return ticket_spin_##op(lock);                                  \
+}

...do you know what impact this inlined static key check has on the
kernel size?

No, I'll check, thanks.

So I have just checked the size of the jump table section:

* defconfig:

- ticket: 26928 bytes
- combo: 28320 bytes

So that's a ~5% increase.

* ubuntu config

- ticket: 107840 bytes
- combo: 174752 bytes

And that's a ~62% increase.

This is the ELF size difference between ticket and combo spinlocks:

* ticket: 776915592 bytes
* combo: 786958968 bytes

So that's an increase of ~1.3% on the ELF.

And the .text section size:

* ticket: 12290960 bytes
* combo: 12366644 bytes

And that's a ~0.6% increase!

Finally, I'd say the impact is very limited :)

Thanks for checking!

drew