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Message-ID: <1cb05816-6f2f-4940-8051-b645f9f52c14@ghiti.fr>
Date: Mon, 3 Jun 2024 13:34:41 +0200
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Subject: Re: [PATCH 7/7] riscv: Add qspinlock support based on Zabha extension
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To: Guo Ren <guoren@kernel.org>
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>, Jonathan Corbet
 <corbet@lwn.net>, Paul Walmsley <paul.walmsley@sifive.com>,
 Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
 Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
 Boqun Feng <boqun.feng@gmail.com>, Arnd Bergmann <arnd@arndb.de>,
 Leonardo Bras <leobras@redhat.com>, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
 linux-arch@vger.kernel.org
References: <20240528151052.313031-1-alexghiti@rivosinc.com>
 <20240528151052.313031-8-alexghiti@rivosinc.com>
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 <b6b6d273-4d57-4611-8c67-1a67d443b7d0@ghiti.fr>
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From: Alexandre Ghiti <alex@ghiti.fr>
In-Reply-To: <CAJF2gTTiz_gkaA8OVkZynKUAFHFGij1W97qVSS3RKyMK5vkpdw@mail.gmail.com>
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On 03/06/2024 13:28, Guo Ren wrote:
> On Mon, Jun 3, 2024 at 5:49 PM Alexandre Ghiti <alex@ghiti.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@rivosinc.com> wrote:
>>>> Hi Guo,
>>>>
>>>> On Wed, May 29, 2024 at 11:24 AM Guo Ren <guoren@kernel.org> wrote:
>>>>> On Tue, May 28, 2024 at 11:18 PM Alexandre Ghiti <alexghiti@rivosinc.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.


I understand qspinlocks require forward progress and that your company's 
LR/SC implementations provide such guarantee, I'm not arguing against 
your new extension proposal.

It seemed to me in your previous message that you implied that when 
NR_CPUS > 16k, we should not use qspinlocks. My question was: "Don't 
Zacas provide such guarantee"? I think it does, so qspinlocks should 
actually depend on Zabha *and* Zacas. Is that correct to you?

Let me know if I misunderstood something again.

Thanks,

Alex


>
>
>> 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@rivosinc.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
>>>
>
>