Re: [PATCH v2 10/10] riscv: Add qspinlock support based on Zabha extension

Guo Ren
2024-07-07T02:21:04Z

On Wed, Jun 26, 2024 at 9:14 PM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
>
> 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 is 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
That's not true; we treat spinlock as qspinlock at first.
qspinlock_unlock would make the lock value zero (clean), but
ticket_lock would make a dirty one. (I've spent much time on this
mechanism, and you've preserved it in this patch.) So, making the
qspinlock -> ticket_lock change point safe until sched_init() is late
enough to make alternatives. The key problem of alternative
implementation is tough coding because you can't reuse the C code. The
whole ticket_lock must be rewritten in asm and include the qspinlock
fast path.

I think we should discuss some points before continuing the patch:
1. Using alternative mechanisms for combo spinlock
2. Using three Kconfigs for
ticket_spinlock/queune_spinlock/combo_spinlock during compile stage.
3. The forward progress guarantee requirement is written in
qspinlock.h comment. That is not about our CAS/BHA.

> 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/ [1]
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> .../locking/queued-spinlocks/arch-support.txt | 2 +-
> arch/riscv/Kconfig | 10 +++++
> arch/riscv/include/asm/Kbuild | 4 +-
> arch/riscv/include/asm/spinlock.h | 39 +++++++++++++++++++
> arch/riscv/kernel/setup.c | 21 ++++++++++
> include/asm-generic/qspinlock.h | 2 +
> include/asm-generic/ticket_spinlock.h | 2 +
> 7 files changed, 78 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 0bbaec0444d0..c2ba673e58ca 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -72,6 +72,7 @@ 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
> select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
> select BUILDTIME_TABLE_SORT if MMU
> select CLINT_TIMER if RISCV_M_MODE
> @@ -482,6 +483,15 @@ config NODES_SHIFT
> Specify the maximum number of NUMA Nodes available on the target
> system. Increases memory reserved to accommodate various tables.
>
> +config RISCV_COMBO_SPINLOCKS
> + bool "Using combo spinlock"
> + depends on SMP && MMU && TOOLCHAIN_HAS_ZABHA
> + select ARCH_USE_QUEUED_SPINLOCKS
> + default y
> + help
> + Embed both queued spinlock and ticket lock so that the spinlock
> + implementation can be chosen at runtime.
> +

COMBO SPINLOCK has side effects, which would expand spinlock code size
a lot. Ref: ARCH_INLINE_SPIN_LOCK

So, we shouldn't remove the three configs' selection.

+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
+ select ARCH_USE_QUEUED_SPINLOCKS
+ help
+ Make sure your micro arch give cmpxchg/xchg forward progress
+ guarantee. Otherwise, stay at ticket-lock.
+
+config RISCV_COMBO_SPINLOCKS
+ bool "Using combo spinlock"
+ depends on SMP && MMU
+ select ARCH_USE_QUEUED_SPINLOCKS
+ help
+ Select queued spinlock or ticket-lock by cmdline.
+endchoice
+

> config RISCV_ALTERNATIVE
> bool
> depends on !XIP_KERNEL
> diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
> index 504f8b7e72d4..ad72f2bd4cc9 100644
> --- a/arch/riscv/include/asm/Kbuild
> +++ b/arch/riscv/include/asm/Kbuild
> @@ -2,10 +2,12 @@
> 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..4856d50006f2
> --- /dev/null
> +++ b/arch/riscv/include/asm/spinlock.h
> @@ -0,0 +1,39 @@
> +/* 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>
> +
> +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); \
> +}
> +
> +SPINLOCK_BASE_DECLARE(lock, void, arch_spinlock_t *)
> +SPINLOCK_BASE_DECLARE(unlock, void, arch_spinlock_t *)
> +SPINLOCK_BASE_DECLARE(is_locked, int, arch_spinlock_t *)
> +SPINLOCK_BASE_DECLARE(is_contended, int, arch_spinlock_t *)
> +SPINLOCK_BASE_DECLARE(trylock, bool, arch_spinlock_t *)
> +SPINLOCK_BASE_DECLARE(value_unlocked, int, arch_spinlock_t)
> +
> +#else
> +
> +#include <asm/ticket_spinlock.h>
> +
> +#endif
> +
> +#include <asm/qrwlock.h>
> +
> +#endif /* __ASM_RISCV_SPINLOCK_H */
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 4f73c0ae44b2..929bccd4c9e5 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -244,6 +244,26 @@ static void __init parse_dtb(void)
> #endif
> }
>
> +DEFINE_STATIC_KEY_TRUE(qspinlock_key);
> +EXPORT_SYMBOL(qspinlock_key);
> +
> +static void __init riscv_spinlock_init(void)
> +{
> + asm goto(ALTERNATIVE("j %[no_zacas]", "nop", 0, RISCV_ISA_EXT_ZACAS, 1)
> + : : : : no_zacas);
> + asm goto(ALTERNATIVE("nop", "j %[qspinlock]", 0, RISCV_ISA_EXT_ZABHA, 1)
> + : : : : qspinlock);
The requirement of qspinlock concerns the forward progress guarantee
in micro-arch design, which includes cmpxchg_loop & xchg_tail. So, I
don't think these features have a relationship with Qspinlock.

If your machine doesn't have enough stickiness for a young exclusive
cacheline, fall back to ticket_lock.

> +
> +no_zacas:
> + static_branch_disable(&qspinlock_key);
> + pr_info("Ticket spinlock: enabled\n");
> +
> + return;
> +
> +qspinlock:
> + pr_info("Queued spinlock: enabled\n");
> +}
> +
> extern void __init init_rt_signal_env(void);
>
> void __init setup_arch(char **cmdline_p)
> @@ -295,6 +315,7 @@ void __init setup_arch(char **cmdline_p)
> riscv_set_dma_cache_alignment();
>
> riscv_user_isa_enable();
> + riscv_spinlock_init();
> }
>
> bool arch_cpu_is_hotpluggable(int cpu)
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index 0655aa5b57b2..bf47cca2c375 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -136,6 +136,7 @@ static __always_inline bool virt_spin_lock(struct qspinlock *lock)
> }
> #endif
>
> +#ifndef __no_arch_spinlock_redefine
> /*
> * Remapping spinlock architecture specific functions to the corresponding
> * queued spinlock functions.
> @@ -146,5 +147,6 @@ static __always_inline bool virt_spin_lock(struct qspinlock *lock)
> #define arch_spin_lock(l) queued_spin_lock(l)
> #define arch_spin_trylock(l) queued_spin_trylock(l)
> #define arch_spin_unlock(l) queued_spin_unlock(l)
> +#endif
>
> #endif /* __ASM_GENERIC_QSPINLOCK_H */
> diff --git a/include/asm-generic/ticket_spinlock.h b/include/asm-generic/ticket_spinlock.h
> index cfcff22b37b3..325779970d8a 100644
> --- a/include/asm-generic/ticket_spinlock.h
> +++ b/include/asm-generic/ticket_spinlock.h
> @@ -89,6 +89,7 @@ static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock)
> return (s16)((val >> 16) - (val & 0xffff)) > 1;
> }
>
> +#ifndef __no_arch_spinlock_redefine
> /*
> * Remapping spinlock architecture specific functions to the corresponding
> * ticket spinlock functions.
> @@ -99,5 +100,6 @@ static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock)
> #define arch_spin_lock(l) ticket_spin_lock(l)
> #define arch_spin_trylock(l) ticket_spin_trylock(l)
> #define arch_spin_unlock(l) ticket_spin_unlock(l)
> +#endif
>
> #endif /* __ASM_GENERIC_TICKET_SPINLOCK_H */
> --
> 2.39.2
>

--
Best Regards
Guo Ren