Re: [PATCH V12 06/14] riscv: qspinlock: Introduce combo spinlock

[Leonardo Bras <leobras@redhat.com>]

On Mon, Dec 25, 2023 at 07:58:39AM -0500, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
> 
> Combo spinlock could support queued and ticket in one Linux Image and
> select them during boot time via command line. Here is the func
> size (Bytes) comparison table below:
> 
> TYPE			: COMBO | TICKET | QUEUED
> arch_spin_lock		: 106	| 60     | 50
> arch_spin_unlock	: 54    | 36     | 26
> arch_spin_trylock	: 110   | 72     | 54
> arch_spin_is_locked	: 48    | 34     | 20
> arch_spin_is_contended	: 56    | 40     | 24
> rch_spin_value_unlocked	: 48    | 34     | 24
> 
> One example of disassemble combo arch_spin_unlock:
>   <+14>:    nop                # detour slot
>   <+18>:    fence   rw,w       --+-> queued_spin_unlock
>   <+22>:    sb      zero,0(a4) --+   (2 instructions)
>   <+26>:    ld      s0,8(sp)
>   <+28>:    addi    sp,sp,16
>   <+30>:    ret
>   <+32>:    lw      a5,0(a4)   --+-> ticket_spin_unlock
>   <+34>:    sext.w  a5,a5        |   (7 instructions)
>   <+36>:    fence   rw,w         |
>   <+40>:    addiw   a5,a5,1      |
>   <+42>:    slli    a5,a5,0x30   |
>   <+44>:    srli    a5,a5,0x30   |
>   <+46>:    sh      a5,0(a4)   --+
>   <+50>:    ld      s0,8(sp)
>   <+52>:    addi    sp,sp,16
>   <+54>:    ret
> The qspinlock is smaller and faster than ticket-lock when all are in a
> fast path.
> 
> The combo spinlock could provide a compatible Linux Image for different
> micro-arch designs that have/haven't forward progress guarantee. Use
> command line options to select between qspinlock and ticket-lock, and
> the default is ticket-lock.
> 
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> ---
>  .../admin-guide/kernel-parameters.txt         |  2 +
>  arch/riscv/Kconfig                            |  9 +++-
>  arch/riscv/include/asm/spinlock.h             | 48 +++++++++++++++++++
>  arch/riscv/kernel/setup.c                     | 34 +++++++++++++
>  include/asm-generic/qspinlock.h               |  2 +
>  include/asm-generic/ticket_spinlock.h         |  2 +
>  6 files changed, 96 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 65731b060e3f..2ac9f1511774 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4753,6 +4753,8 @@
>  			[KNL] Number of legacy pty's. Overwrites compiled-in
>  			default number.
>  
> +	qspinlock	[RISCV] Use native qspinlock.
> +
>  	quiet		[KNL] Disable most log messages
>  
>  	r128=		[HW,DRM]
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index f345df0763b2..b7673c5c0997 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -434,7 +434,7 @@ config NODES_SHIFT
>  
>  choice
>  	prompt "RISC-V spinlock type"
> -	default RISCV_TICKET_SPINLOCKS
> +	default RISCV_COMBO_SPINLOCKS
>  
>  config RISCV_TICKET_SPINLOCKS
>  	bool "Using ticket spinlock"
> @@ -446,6 +446,13 @@ config RISCV_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
> diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
> index 98a3da4b1056..d07643c07aae 100644
> --- a/arch/riscv/include/asm/spinlock.h
> +++ b/arch/riscv/include/asm/spinlock.h
> @@ -7,12 +7,60 @@
>  #define _Q_PENDING_LOOPS	(1 << 9)
>  #endif
>  
> +#ifdef CONFIG_RISCV_COMBO_SPINLOCKS
> +#define __no_arch_spinlock_redefine
> +#include <asm/ticket_spinlock.h>
> +#include <asm/qspinlock.h>
> +#include <linux/jump_label.h>
> +
> +DECLARE_STATIC_KEY_TRUE(combo_qspinlock_key);
> +
> +#define COMBO_SPINLOCK_BASE_DECLARE(op)					\
> +static __always_inline void arch_spin_##op(arch_spinlock_t *lock)	\
> +{									\
> +	if (static_branch_likely(&combo_qspinlock_key))			\
> +		queued_spin_##op(lock);					\
> +	else								\
> +		ticket_spin_##op(lock);					\
> +}
> +COMBO_SPINLOCK_BASE_DECLARE(lock)
> +COMBO_SPINLOCK_BASE_DECLARE(unlock)
> +
> +#define COMBO_SPINLOCK_IS_DECLARE(op)					\
> +static __always_inline int arch_spin_##op(arch_spinlock_t *lock)	\
> +{									\
> +	if (static_branch_likely(&combo_qspinlock_key))			\
> +		return queued_spin_##op(lock);				\
> +	else								\
> +		return ticket_spin_##op(lock);				\
> +}
> +COMBO_SPINLOCK_IS_DECLARE(is_locked)
> +COMBO_SPINLOCK_IS_DECLARE(is_contended)
> +
> +static __always_inline bool arch_spin_trylock(arch_spinlock_t *lock)
> +{
> +	if (static_branch_likely(&combo_qspinlock_key))
> +		return queued_spin_trylock(lock);
> +	else
> +		return ticket_spin_trylock(lock);
> +}
> +
> +static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> +{
> +	if (static_branch_likely(&combo_qspinlock_key))
> +		return queued_spin_value_unlocked(lock);
> +	else
> +		return ticket_spin_value_unlocked(lock);
> +}
> +

Hello Guo Ren,

The above is much better than v11, but can be improved as I mentioned in 
my reply in v11. Okay, I noticed there is a type issue: some return int, 
others return bool while some return void, but it can be improved:

+#define COMBO_SPINLOCK_DECLARE(op, type)                             \
+static __always_inline type arch_spin_##op(arch_spinlock_t *lock)    \
+{                                                                    \
+     if (static_branch_likely(&combo_qspinlock_key))                 \
+             return queued_spin_##op(lock);                          \
+     else                                                            \
+             return ticket_spin_##op(lock);                          \
+}
+
+COMBO_SPINLOCK_DECLARE(lock, void)
+COMBO_SPINLOCK_DECLARE(unlock, void)
+COMBO_SPINLOCK_DECLARE(is_locked, int)
+COMBO_SPINLOCK_DECLARE(is_contended, int)
+COMBO_SPINLOCK_DECLARE(value_unlocked, int)
+COMBO_SPINLOCK_DECLARE(trylock, bool)

===
IIRC it's legal to return a void f1() from a void f2():

void f1() {}

void f2() {
	return f1(); /* <- IIRC it's legal :)*/
}
===

> +#else /* CONFIG_RISCV_COMBO_SPINLOCKS */
>  #ifdef CONFIG_QUEUED_SPINLOCKS
>  #include <asm/qspinlock.h>
>  #else
>  #include <asm/ticket_spinlock.h>
>  #endif
>  
> +#endif /* CONFIG_RISCV_COMBO_SPINLOCKS */
>  #include <asm/qrwlock.h>
>  
>  #endif /* __ASM_RISCV_SPINLOCK_H */
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 535a837de55d..d9072a59831c 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -246,6 +246,37 @@ static void __init parse_dtb(void)
>  #endif
>  }
>  
> +#ifdef CONFIG_RISCV_COMBO_SPINLOCKS
> +static bool enable_qspinlock __ro_after_init;
> +static int __init queued_spinlock_setup(char *p)
> +{
> +	enable_qspinlock = true;
> +
> +	return 0;
> +}
> +early_param("qspinlock", queued_spinlock_setup);
> +
> +/*
> + * Ticket-lock would dirty the lock value, so force qspinlock at
> + * first and switch to ticket-lock later.
> + *  - key is true : qspinlock -> qspinlock (no change)
> + *  - key is false: qspinlock -> ticket-lock
> + *    (No ticket-lock -> qspinlock)
> + */
> +DEFINE_STATIC_KEY_TRUE(combo_qspinlock_key);
> +EXPORT_SYMBOL(combo_qspinlock_key);
> +
> +static void __init riscv_spinlock_init(void)
> +{
> +	if (!enable_qspinlock) {
> +		static_branch_disable(&combo_qspinlock_key);
> +		pr_info("Ticket spinlock: enabled\n");
> +	} else {
> +		pr_info("Queued spinlock: enabled\n");
> +	}
> +}
> +#endif
> +
>  extern void __init init_rt_signal_env(void);
>  
>  void __init setup_arch(char **cmdline_p)
> @@ -297,6 +328,9 @@ void __init setup_arch(char **cmdline_p)
>  	riscv_set_dma_cache_alignment();
>  
>  	riscv_user_isa_enable();
> +#ifdef CONFIG_RISCV_COMBO_SPINLOCKS
> +	riscv_spinlock_init();
> +#endif
>  }
>  
>  static int __init topology_init(void)
> 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 */
> -- 
> 2.40.1
> 


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