Re: [PATCH] riscv: mm: try VMA lock-based page fault handling first

[Kefeng Wang]

On 2023/5/24 0:59, Jisheng Zhang wrote:
> Attempt VMA lock-based page fault handling first, and fall back to the
> existing mmap_lock-based handling if that fails.
>
> A simple running the ebizzy benchmark on Lichee Pi 4A shows that
> PER_VMA_LOCK can improve the ebizzy benchmark by about 32.68%. In
> theory, the more CPUs, the bigger improvement, but I don't have any
> HW platform which has more than 4 CPUs.
>
> This is the riscv variant of "x86/mm: try VMA lock-based page fault
> handling first".
>
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> ---
> Any performance numbers are welcome! Especially the numbers on HW
> platforms with 8 or more CPUs.
>
>   arch/riscv/Kconfig    |  1 +
>   arch/riscv/mm/fault.c | 33 +++++++++++++++++++++++++++++++++
>   2 files changed, 34 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 62e84fee2cfd..b958f67f9a12 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -42,6 +42,7 @@ config RISCV
>   	select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
>   	select ARCH_SUPPORTS_HUGETLBFS if MMU
>   	select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
> +	select ARCH_SUPPORTS_PER_VMA_LOCK if MMU

no need if mmu，  see PER_VMA_LOCK


config PER_VMA_LOCK
        bool "allow VMA lock-based page fault"
        def_bool y
        depends on ARCH_SUPPORTS_PER_VMA_LOCK && MMU && SMP

Reviewed-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>

>   	select ARCH_USE_MEMTEST
>   	select ARCH_USE_QUEUED_RWLOCKS
>   	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index 8685f85a7474..eccdddf26f4b 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -286,6 +286,36 @@ void handle_page_fault(struct pt_regs *regs)
>   		flags |= FAULT_FLAG_WRITE;
>   	else if (cause == EXC_INST_PAGE_FAULT)
>   		flags |= FAULT_FLAG_INSTRUCTION;
> +#ifdef CONFIG_PER_VMA_LOCK
> +	if (!(flags & FAULT_FLAG_USER))
> +		goto lock_mmap;
> +
> +	vma = lock_vma_under_rcu(mm, addr);
> +	if (!vma)
> +		goto lock_mmap;
> +
> +	if (unlikely(access_error(cause, vma))) {
> +		vma_end_read(vma);
> +		goto lock_mmap;
> +	}
> +
> +	fault = handle_mm_fault(vma, addr, flags | FAULT_FLAG_VMA_LOCK, regs);
> +	vma_end_read(vma);
> +
> +	if (!(fault & VM_FAULT_RETRY)) {
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +		goto done;
> +	}
> +	count_vm_vma_lock_event(VMA_LOCK_RETRY);
> +
> +	if (fault_signal_pending(fault, regs)) {
> +		if (!user_mode(regs))
> +			no_context(regs, addr);
> +		return;
> +	}
> +lock_mmap:
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
>   retry:
>   	mmap_read_lock(mm);
>   	vma = find_vma(mm, addr);
> @@ -355,6 +385,9 @@ void handle_page_fault(struct pt_regs *regs)
>   
>   	mmap_read_unlock(mm);
>   
> +#ifdef CONFIG_PER_VMA_LOCK
> +done:
> +#endif
>   	if (unlikely(fault & VM_FAULT_ERROR)) {
>   		tsk->thread.bad_cause = cause;
>   		mm_fault_error(regs, addr, fault);