On Wed, Jun 26, 2024 at 9:14 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> 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@xxxxxxxxxx/ [1] > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> > --- > .../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
