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Date: Mon, 3 Jun 2024 19:28:11 +0800
From: Guo Ren <guoren@...nel.org>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>, Jonathan Corbet <corbet@....net>,
Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>, Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
Boqun Feng <boqun.feng@...il.com>, Arnd Bergmann <arnd@...db.de>, Leonardo Bras <leobras@...hat.com>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 7/7] riscv: Add qspinlock support based on Zabha extension
On Mon, Jun 3, 2024 at 5:49 PM Alexandre Ghiti <alex@...ti.fr> wrote:
>
> Hi Guo,
>
> On 31/05/2024 15:10, Guo Ren wrote:
> > On Wed, May 29, 2024 at 9:03 PM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> >> Hi Guo,
> >>
> >> On Wed, May 29, 2024 at 11:24 AM Guo Ren <guoren@...nel.org> wrote:
> >>> On Tue, May 28, 2024 at 11:18 PM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> >>>> In order to produce a generic kernel, a user can select
> >>>> CONFIG_QUEUED_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
> >>>> implementation because the ticket spinlock implementation would pollute
> >>>> the spinlock value, so let's use static keys.
> > Zabha is not a prerequisite for qspinlock; the prerequisite for
> > qspinlock is the *forward progress guarantee* in the atomic operation
> > loop during intense contention. Even with Zabha enabled to meet the
> > requirements of xchg_tail, that still only applies when the number of
> > CPUs is less than 16K. The qspinlock uses cmpxchg loop instead of
> > xchg_tail when the number of cores is more than 16K. Thus, hardware
> > support for Zabha does not equate to the safe use of qspinlock.
>
>
> But if we have Zacas to implement cmpxchg(), we still provide the
> "forward progress guarantee" then right? Let me know if I missed something.
The qspinlock needs a "forward progress guarantee," not Zacas, and
Zabha could give a guarantee to qspinlock xchg_tail (CPUs < 16K) with
AMOSWAP.H instruction. But, using "LR/SC pairs" also could give enough
fwd guarantee that depends on the micro-arch design of the riscv core.
I think the help of AMO instead of LR/SC is it could off-load AMO
operations from LSU to CIU(Next Level Cache or Interconnect), which
gains better performance. "LR/SC pairs" only provide Near-Atomic, but
AMO gives Far-Atomic additionally.
>
> Thanks,
>
> Alex
>
>
> >
> > Therefore, I would like to propose a new ISA extension: Zafpg(Atomic
> > Forward Progress Guarantee). If RISC-V vendors can ensure the progress
> > of LR/SC or CMPXCHG LOOP at the microarchitectural level or if cache
> > lines are sufficiently sticky, they could then claim support for this
> > extension. Linux could then select different spinlock implementations
> > based on this extension's support or not.
> >
> >>>> 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@...osinc.com>
> >>>> ---
> >>>> .../locking/queued-spinlocks/arch-support.txt | 2 +-
> >>>> arch/riscv/Kconfig | 1 +
> >>>> arch/riscv/include/asm/Kbuild | 4 +-
> >>>> arch/riscv/include/asm/spinlock.h | 39 +++++++++++++++++++
> >>>> arch/riscv/kernel/setup.c | 18 +++++++++
> >>>> include/asm-generic/qspinlock.h | 2 +
> >>>> include/asm-generic/ticket_spinlock.h | 2 +
> >>>> 7 files changed, 66 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 184a9edb04e0..ccf1703edeb9 100644
> >>>> --- a/arch/riscv/Kconfig
> >>>> +++ b/arch/riscv/Kconfig
> >>>> @@ -59,6 +59,7 @@ config RISCV
> >>>> select ARCH_SUPPORTS_SHADOW_CALL_STACK if HAVE_SHADOW_CALL_STACK
> >>>> select ARCH_USE_MEMTEST
> >>>> select ARCH_USE_QUEUED_RWLOCKS
> >>>> + select ARCH_USE_QUEUED_SPINLOCKS if TOOLCHAIN_HAS_ZABHA
> >>> Using qspinlock or not depends on real hardware capabilities, not the
> >>> compiler flag. That's why I introduced combo-spinlock, ticket-spinlock
> >>> & qspinlock three Kconfigs, and the combo-spinlock would compat all
> >>> hardware platforms but waste some qspinlock code size.
> >> You're right, and I think your comment matches what Conor mentioned
> >> about the lack of clarity with some extensions: TOOLCHAIN_HAS_ZABHA
> >> will allow a platform with Zabha capability to use qspinlocks. But if
> >> the hardware does not, it will fallback to the ticket spinlocks.
> >>
> >> But I agree that looking at the config alone may be misleading, even
> >> though it will work as expected at runtime. So I agree with you:
> >> unless anyone is strongly against the combo spinlocks, I will do what
> >> you suggest and add them.
> >>
> >> Thanks again for your initial work,
> >>
> >> Alex
> >>
> >>>> select ARCH_USES_CFI_TRAPS if CFI_CLANG
> >>>> select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH if SMP && MMU
> >>>> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> >>>> 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..e00429ac20ed
> >>>> --- /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_QUEUED_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..31ce75522fd4 100644
> >>>> --- a/arch/riscv/kernel/setup.c
> >>>> +++ b/arch/riscv/kernel/setup.c
> >>>> @@ -244,6 +244,23 @@ 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("nop", "j %[qspinlock]", 0, RISCV_ISA_EXT_ZABHA, 1)
> >>>> + : : : : qspinlock);
> >>>> +
> >>>> + 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 +312,7 @@ 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
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