On Wed, 22 May 2019 17:09:37 +0200 "Uladzislau Rezki (Sony)" <ure...@gmail.com>
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.
