Re: [PATCH] mm/vmalloc: lock contention optimization under multi-threading

[Uladzislau Rezki]

On Tue, Feb 06, 2024 at 10:30:59PM -0500, rulinhuang wrote:
> When allocating a new memory area where the mapping address range is
> known, it is observed that the vmap_area lock is acquired twice.
> The first acquisition occurs in the alloc_vmap_area() function when
> inserting the vm area into the vm mapping red-black tree. The second
> acquisition occurs in the setup_vmalloc_vm() function when updating the
> properties of the vm, such as flags and address, etc.
> Combine these two operations together in alloc_vmap_area(), which
> improves scalability when the vmap_area lock is contended. By doing so,
> the need to acquire the lock twice can also be eliminated.
> With the above change, tested on intel icelake platform(160 vcpu, kernel
> v6.7), a 6% performance improvement and a 7% reduction in overall
> spinlock hotspot are gained on
> stress-ng/pthread(https://github.com/ColinIanKing/stress-ng), which is
> the stress test of thread creations.
> 
> Reviewed-by: "Chen, Tim C" <tim.c.chen@xxxxxxxxx>
> Reviewed-by: "King, Colin" <colin.king@xxxxxxxxx>
> Signed-off-by: rulinhuang <rulin.huang@xxxxxxxxx>
> ---
>  mm/vmalloc.c | 31 +++++++++++++++----------------
>  1 file changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d12a17fc0c171..3b1f616e8ecf0 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1577,13 +1577,13 @@ preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)
>  
>  /*
>   * Allocate a region of KVA of the specified size and alignment, within the
> - * vstart and vend.
> + * vstart and vend. If vm is passed in, the two will also be bound.
>   */
>  static struct vmap_area *alloc_vmap_area(unsigned long size,
>  				unsigned long align,
>  				unsigned long vstart, unsigned long vend,
>  				int node, gfp_t gfp_mask,
> -				unsigned long va_flags)
> +				unsigned long va_flags, struct vm_struct *vm)
>  {
>  	struct vmap_area *va;
>  	unsigned long freed;
> @@ -1627,9 +1627,12 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  
>  	va->va_start = addr;
>  	va->va_end = addr + size;
> -	va->vm = NULL;
> +	va->vm = vm;
>  	va->flags = va_flags;
>  
> +	if (vm != NULL)
> +		vm->addr = (void *)addr;
> +
>  	spin_lock(&vmap_area_lock);
>  	insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
>  	spin_unlock(&vmap_area_lock);
> @@ -2039,7 +2042,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  	va = alloc_vmap_area(VMAP_BLOCK_SIZE, VMAP_BLOCK_SIZE,
>  					VMALLOC_START, VMALLOC_END,
>  					node, gfp_mask,
> -					VMAP_RAM|VMAP_BLOCK);
> +					VMAP_RAM|VMAP_BLOCK,
> +					NULL);
>  	if (IS_ERR(va)) {
>  		kfree(vb);
>  		return ERR_CAST(va);
> @@ -2394,7 +2398,8 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
>  		struct vmap_area *va;
>  		va = alloc_vmap_area(size, PAGE_SIZE,
>  				VMALLOC_START, VMALLOC_END,
> -				node, GFP_KERNEL, VMAP_RAM);
> +				node, GFP_KERNEL, VMAP_RAM,
> +				NULL);
>  		if (IS_ERR(va))
>  			return NULL;
>  
> @@ -2548,14 +2553,6 @@ static inline void setup_vmalloc_vm_locked(struct vm_struct *vm,
>  	va->vm = vm;
>  }
>  
> -static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
> -			      unsigned long flags, const void *caller)
> -{
> -	spin_lock(&vmap_area_lock);
> -	setup_vmalloc_vm_locked(vm, va, flags, caller);
> -	spin_unlock(&vmap_area_lock);
> -}
> -
>  static void clear_vm_uninitialized_flag(struct vm_struct *vm)
>  {
>  	/*
> @@ -2592,14 +2589,16 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
>  	if (!(flags & VM_NO_GUARD))
>  		size += PAGE_SIZE;
>  
> -	va = alloc_vmap_area(size, align, start, end, node, gfp_mask, 0);
> +	area->flags = flags;
> +	area->caller = caller;
> +	area->size = size;
> +
> +	va = alloc_vmap_area(size, align, start, end, node, gfp_mask, 0, area);
>  	if (IS_ERR(va)) {
>  		kfree(area);
>  		return NULL;
>  	}
>  
> -	setup_vmalloc_vm(area, va, flags, caller);
> -
>  	/*
>
Thank you for improving this! That was in my radar quite a long time ago :)

Some comments though. I think that such "partial" way of initializing of
"vm_struct" or "vm" is not optimal because it becomes spread between two
places.

Also, i think that we should not remove the setup_vmalloc_vm() function,
instead we can move it in a place where the VA is not inserted to the
tree, i.e. to initialize before insert.

The "locked" variant can be renamed to setup_vmalloc_vm(). 

Also, could you please post how you tested this patch and stress-ng
figures? This is really good for other people and folk to know.

Thanks!

--
Uladzislau Rezki