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

Guo Ren
2024-07-08T11:52:06Z

On Sun, Jul 7, 2024 at 10:20 AM Guo Ren <guoren@kernel.org> wrote:
>
> 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.

Could we use "Ziccrse: Main memory supports forward progress on LR/SC
sequences" for qspinlock selection?

+ asm goto(ALTERNATIVE("nop", "j %[qspinlock]", 0,
RISCV_ISA_EXT_ZICCRSE, 1)
: : : : qspinlock);

[1] https://github.com/riscv/riscv-profiles/blob/main/src/rva23-profile.adoc

>
> > +
> > +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

--
Best Regards
Guo Ren