Re: [PATCH V11 05/17] riscv: qspinlock: Add basic queued_spinlock support

[Leonardo Bras <leobras@redhat.com>]

On Sun, Sep 10, 2023 at 04:28:59AM -0400, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
> 
> The requirements of qspinlock have been documented by commit:
> a8ad07e5240c ("asm-generic: qspinlock: Indicate the use of mixed-size
> atomics").
> 
> Although RISC-V ISA gives out a weaker forward guarantee LR/SC, which
> doesn't satisfy the requirements of qspinlock above, it won't prevent
> some riscv vendors from implementing a strong fwd guarantee LR/SC in
> microarchitecture to match xchg_tail requirement. T-HEAD C9xx processor
> is the one.
> 
> We've tested the patch on SOPHGO sg2042 & th1520 and passed the stress
> test on Fedora & Ubuntu & OpenEuler ... Here is the performance
> comparison between qspinlock and ticket_lock on sg2042 (64 cores):
> 
> sysbench test=threads threads=32 yields=100 lock=8 (+13.8%):
>   queued_spinlock 0.5109/0.00
>   ticket_spinlock 0.5814/0.00
> 
> perf futex/hash (+6.7%):
>   queued_spinlock 1444393 operations/sec (+- 0.09%)
>   ticket_spinlock 1353215 operations/sec (+- 0.15%)
> 
> perf futex/wake-parallel (+8.6%):
>   queued_spinlock (waking 1/64 threads) in 0.0253 ms (+-2.90%)
>   ticket_spinlock (waking 1/64 threads) in 0.0275 ms (+-3.12%)
> 
> perf futex/requeue (+4.2%):
>   queued_spinlock Requeued 64 of 64 threads in 0.0785 ms (+-0.55%)
>   ticket_spinlock Requeued 64 of 64 threads in 0.0818 ms (+-4.12%)
> 
> System Benchmarks (+6.4%)
>   queued_spinlock:
>     System Benchmarks Index Values               BASELINE       RESULT    INDEX
>     Dhrystone 2 using register variables         116700.0  628613745.4  53865.8
>     Double-Precision Whetstone                       55.0     182422.8  33167.8
>     Execl Throughput                                 43.0      13116.6   3050.4
>     File Copy 1024 bufsize 2000 maxblocks          3960.0    7762306.2  19601.8
>     File Copy 256 bufsize 500 maxblocks            1655.0    3417556.8  20649.9
>     File Copy 4096 bufsize 8000 maxblocks          5800.0    7427995.7  12806.9
>     Pipe Throughput                               12440.0   23058600.5  18535.9
>     Pipe-based Context Switching                   4000.0    2835617.7   7089.0
>     Process Creation                                126.0      12537.3    995.0
>     Shell Scripts (1 concurrent)                     42.4      57057.4  13456.9
>     Shell Scripts (8 concurrent)                      6.0       7367.1  12278.5
>     System Call Overhead                          15000.0   33308301.3  22205.5
>                                                                        ========
>     System Benchmarks Index Score                                       12426.1
> 
>   ticket_spinlock:
>     System Benchmarks Index Values               BASELINE       RESULT    INDEX
>     Dhrystone 2 using register variables         116700.0  626541701.9  53688.2
>     Double-Precision Whetstone                       55.0     181921.0  33076.5
>     Execl Throughput                                 43.0      12625.1   2936.1
>     File Copy 1024 bufsize 2000 maxblocks          3960.0    6553792.9  16550.0
>     File Copy 256 bufsize 500 maxblocks            1655.0    3189231.6  19270.3
>     File Copy 4096 bufsize 8000 maxblocks          5800.0    7221277.0  12450.5
>     Pipe Throughput                               12440.0   20594018.7  16554.7
>     Pipe-based Context Switching                   4000.0    2571117.7   6427.8
>     Process Creation                                126.0      10798.4    857.0
>     Shell Scripts (1 concurrent)                     42.4      57227.5  13497.1
>     Shell Scripts (8 concurrent)                      6.0       7329.2  12215.3
>     System Call Overhead                          15000.0   30766778.4  20511.2
>                                                                        ========
>     System Benchmarks Index Score                                       11670.7
> 
> The qspinlock has a significant improvement on SOPHGO SG2042 64
> cores platform than the ticket_lock.
> 
> Signed-off-by: Guo Ren <guoren@kernel.org>
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> ---
>  arch/riscv/Kconfig                | 16 ++++++++++++++++
>  arch/riscv/include/asm/Kbuild     |  3 ++-
>  arch/riscv/include/asm/spinlock.h | 17 +++++++++++++++++
>  3 files changed, 35 insertions(+), 1 deletion(-)
>  create mode 100644 arch/riscv/include/asm/spinlock.h
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 2c346fe169c1..7f39bfc75744 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -471,6 +471,22 @@ config NODES_SHIFT
>  	  Specify the maximum number of NUMA Nodes available on the target
>  	  system.  Increases memory reserved to accommodate various tables.
>  
> +choice
> +	prompt "RISC-V spinlock type"
> +	default RISCV_TICKET_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 LL/SC has a strong forward progress guarantee.
> +	  Otherwise, stay at ticket-lock.
> +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..a0dc85e4a754 100644
> --- a/arch/riscv/include/asm/Kbuild
> +++ b/arch/riscv/include/asm/Kbuild
> @@ -2,10 +2,11 @@
>  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

IIUC here you take the asm-generic/spinlock.h (which defines arch_spin_*()) 
and include the asm-generic headers of mcs_spinlock and qspinlock. 

In this case, the qspinlock.h will provide the arch_spin_*() interfaces, 
which seems the oposite of the above description (ticket spinlocks being 
the standard).

Shouldn't ticket-spinlock.h also get included here?
(Also, I am probably missing something, as I dont' see the use of 
mcs_spinlock here.)

>  generic-y += spinlock_types.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..c644a92d4548
> --- /dev/null
> +++ b/arch/riscv/include/asm/spinlock.h
> @@ -0,0 +1,17 @@
> +/* 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)
> +#endif

Any reason the above define couldn't be merged on the ifdef below?

> +
> +#ifdef CONFIG_QUEUED_SPINLOCKS
> +#include <asm/qspinlock.h>
> +#include <asm/qrwlock.h>
> +#else
> +#include <asm-generic/spinlock.h>
> +#endif
> +
> +#endif /* __ASM_RISCV_SPINLOCK_H */
> -- 
> 2.36.1
> 

Thanks!
Leo