Re: [PATCH 2/4] mm/vmap: preload a CPU with one object for split purpose

[Andrew Morton]

On Wed, 22 May 2019 17:09:37 +0200 "Uladzislau Rezki (Sony)" <urezki@xxxxxxxxx> wrote:

> Introduce ne_fit_preload()/ne_fit_preload_end() functions
> for preloading one extra vmap_area object to ensure that
> we have it available when fit type is NE_FIT_TYPE.
> 
> The preload is done per CPU and with GFP_KERNEL permissive
> allocation masks, which allow to be more stable under low
> memory condition and high memory pressure.

What is the reason for this change?  Presumably some workload is
suffering from allocation failures?  Please provide a full description
of when and how this occurs so others can judge the desirability of
this change.

> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -364,6 +364,13 @@ static LIST_HEAD(free_vmap_area_list);
>   */
>  static struct rb_root free_vmap_area_root = RB_ROOT;
>  
> +/*
> + * Preload a CPU with one object for "no edge" split case. The
> + * aim is to get rid of allocations from the atomic context, thus
> + * to use more permissive allocation masks.
> + */
> +static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
> +
>  static __always_inline unsigned long
>  va_size(struct vmap_area *va)
>  {
> @@ -950,9 +957,24 @@ adjust_va_to_fit_type(struct vmap_area *va,
>  		 *   L V  NVA  V R
>  		 * |---|-------|---|
>  		 */
> -		lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
> -		if (unlikely(!lva))
> -			return -1;
> +		lva = __this_cpu_xchg(ne_fit_preload_node, NULL);
> +		if (unlikely(!lva)) {
> +			/*
> +			 * For percpu allocator we do not do any pre-allocation
> +			 * and leave it as it is. The reason is it most likely
> +			 * never ends up with NE_FIT_TYPE splitting. In case of
> +			 * percpu allocations offsets and sizes are aligned to
> +			 * fixed align request, i.e. RE_FIT_TYPE and FL_FIT_TYPE
> +			 * are its main fitting cases.
> +			 *
> +			 * There are few exceptions though, as en example it is

"a few"

s/en/an/

> +			 * a first allocation(early boot up) when we have "one"

s/(/ (/

> +			 * big free space that has to be split.
> +			 */
> +			lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
> +			if (!lva)
> +				return -1;
> +		}
>  
>  		/*
>  		 * Build the remainder.
> @@ -1023,6 +1045,50 @@ __alloc_vmap_area(unsigned long size, unsigned long align,
>  }
>  
>  /*
> + * Preload this CPU with one extra vmap_area object to ensure
> + * that we have it available when fit type of free area is
> + * NE_FIT_TYPE.
> + *
> + * The preload is done in non-atomic context thus, it allows us

s/ thus,/, thus/

> + * to use more permissive allocation masks, therefore to be more

s/, therefore//

> + * stable under low memory condition and high memory pressure.
> + *
> + * If success, it returns zero with preemption disabled. In case
> + * of error, (-ENOMEM) is returned with preemption not disabled.
> + * Note it has to be paired with alloc_vmap_area_preload_end().
> + */
> +static void
> +ne_fit_preload(int *preloaded)
> +{
> +	preempt_disable();
> +
> +	if (!__this_cpu_read(ne_fit_preload_node)) {
> +		struct vmap_area *node;
> +
> +		preempt_enable();
> +		node = kmem_cache_alloc(vmap_area_cachep, GFP_KERNEL);
> +		if (node == NULL) {
> +			*preloaded = 0;
> +			return;
> +		}
> +
> +		preempt_disable();
> +
> +		if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, node))
> +			kmem_cache_free(vmap_area_cachep, node);
> +	}
> +
> +	*preloaded = 1;
> +}

Why not make it do `return preloaded;'?  The
pass-and-return-by-reference seems unnecessary?

Otherwise it all appears solid and sensible.