Re: [PATCH v3] mm/vmalloc: lock contention optimization under multi-threading

[Uladzislau Rezki]

On Tue, Feb 20, 2024 at 10:29:05PM -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>
> ---
> V1 -> V2: Avoided the partial initialization issue of vm and 
> separated insert_vmap_area() from alloc_vmap_area()
> V2 -> V3: Rebased on 6.8-rc5
> ---
>  mm/vmalloc.c | 36 +++++++++++++++++++++---------------
>  1 file changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d12a17fc0c17..768e45f2ed94 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1630,17 +1630,18 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  	va->vm = NULL;
>  	va->flags = va_flags;
>  
> -	spin_lock(&vmap_area_lock);
> -	insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
> -	spin_unlock(&vmap_area_lock);
> -
>  	BUG_ON(!IS_ALIGNED(va->va_start, align));
>  	BUG_ON(va->va_start < vstart);
>  	BUG_ON(va->va_end > vend);
>  
>  	ret = kasan_populate_vmalloc(addr, size);
>  	if (ret) {
> -		free_vmap_area(va);
> +		/*
> +		 * Insert/Merge it back to the free tree/list.
> +		 */
> +		spin_lock(&free_vmap_area_lock);
> +		merge_or_add_vmap_area_augment(va, &free_vmap_area_root, &free_vmap_area_list);
> +		spin_unlock(&free_vmap_area_lock);
>  		return ERR_PTR(ret);
>  	}
>  
> @@ -1669,6 +1670,13 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  	return ERR_PTR(-EBUSY);
>  }
>  
> +static inline void insert_vmap_area_with_lock(struct vmap_area *va)
> +{
> +	spin_lock(&vmap_area_lock);
> +	insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
> +	spin_unlock(&vmap_area_lock);
> +}
> +
>  int register_vmap_purge_notifier(struct notifier_block *nb)
>  {
>  	return blocking_notifier_chain_register(&vmap_notify_list, nb);
> @@ -2045,6 +2053,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  		return ERR_CAST(va);
>  	}
>  
> +	insert_vmap_area_with_lock(va);
> +
>  	vaddr = vmap_block_vaddr(va->va_start, 0);
>  	spin_lock_init(&vb->lock);
>  	vb->va = va;
> @@ -2398,6 +2408,8 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
>  		if (IS_ERR(va))
>  			return NULL;
>  
> +		insert_vmap_area_with_lock(va);
> +
>  		addr = va->va_start;
>  		mem = (void *)addr;
>  	}
> @@ -2538,7 +2550,7 @@ static void vmap_init_free_space(void)
>  	}
>  }
>  
> -static inline void setup_vmalloc_vm_locked(struct vm_struct *vm,
> +static inline void setup_vmalloc_vm(struct vm_struct *vm,
>  	struct vmap_area *va, unsigned long flags, const void *caller)
>  {
>  	vm->flags = flags;
> @@ -2548,14 +2560,6 @@ 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)
>  {
>  	/*
> @@ -2600,6 +2604,8 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
>  
>  	setup_vmalloc_vm(area, va, flags, caller);
>  
> +	insert_vmap_area_with_lock(va);
> +
>
>  	/*
>  	 * Mark pages for non-VM_ALLOC mappings as accessible. Do it now as a
>  	 * best-effort approach, as they can be mapped outside of vmalloc code.
> @@ -4166,7 +4172,7 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
>  	for (area = 0; area < nr_vms; area++) {
>  		insert_vmap_area(vas[area], &vmap_area_root, &vmap_area_list);
>  
> -		setup_vmalloc_vm_locked(vms[area], vas[area], VM_ALLOC,
> +		setup_vmalloc_vm(vms[area], vas[area], VM_ALLOC,
>  				 pcpu_get_vm_areas);
>  	}
>  	spin_unlock(&vmap_area_lock);
> 
> base-commit: b401b621758e46812da61fa58a67c3fd8d91de0d
> -- 
> 2.43.0
> 
Spreading the insert_vmap_area_lock() across several callers, like:
__get_vm_area_node(), new_vmap_block(), vm_map_ram(), etc is not good
approach, simply because it changes the behaviour and people might
miss this point.

Could you please re-spin it on the mm-unstable, because the vmalloc
code was changes a lot? From my side i can check and help you how to
fix it in a better way. Because the v3 should be improved anyaway.

Apparently i have not seen you messages for some reason, i do not
understand why. I started to get emails with below topic:

"Bounce probe for linux-kernel@xxxxxxxxxxxxxxx (no action required)"

--
Uladzislau Rezki