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

[Jisheng Zhang <jszhang@kernel.org>]

On Wed, May 24, 2023 at 01:02:59AM -0400, guoren@kernel.org 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
> Good improvement, I think VMA lock is worth to support in riscv.
> 
> Please give more details about ebizzy, Is it 
> https://github.com/linux-test-project/ltp/blob/master/utils/benchmark/ebizzy-0.3/ebizzy.c
> ?

yeah it's one of ltp benchmark utils.

> 
> > 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".
> >
> 
> How about add Link tag here:
> Link: https://lwn.net/Articles/906852/
> 
> > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> > ---
> > 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
> >  	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
> It's very close to cd7f176aea5f ("arm64/mm: try VMA lock-based page fault 
> handling first"), and I didn't find any problem. So:
> 
> Reviewed-by: Guo Ren <guoren@kernel.org>
> 
> F.Y.I Huacai Chen, maybe he also would be interesting this new feature.
> 
> 
> >  	if (unlikely(fault & VM_FAULT_ERROR)) {
> >  		tsk->thread.bad_cause = cause;
> >  		mm_fault_error(regs, addr, fault);
> > -- 
> > 2.40.1

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