Re: [PATCH 2/9] mm/page_alloc: Add a bulk page allocator

Mel Gorman
2021-04-12T10:59:46Z

On Mon, Apr 12, 2021 at 12:21:42PM +0200, Vlastimil Babka wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 8a3e13277e22..eb547470a7e4 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4965,6 +4965,124 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
> > return true;
> > }
> >
> > +/*
> > + * __alloc_pages_bulk - Allocate a number of order-0 pages to a list
> > + * @gfp: GFP flags for the allocation
> > + * @preferred_nid: The preferred NUMA node ID to allocate from
> > + * @nodemask: Set of nodes to allocate from, may be NULL
> > + * @nr_pages: The number of pages desired on the list
> > + * @page_list: List to store the allocated pages
> > + *
> > + * This is a batched version of the page allocator that attempts to
> > + * allocate nr_pages quickly and add them to a list.
> > + *
> > + * Returns the number of pages on the list.
> > + */
> > +int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> > + nodemask_t *nodemask, int nr_pages,
> > + struct list_head *page_list)
> > +{
> > + struct page *page;
> > + unsigned long flags;
> > + struct zone *zone;
> > + struct zoneref *z;
> > + struct per_cpu_pages *pcp;
> > + struct list_head *pcp_list;
> > + struct alloc_context ac;
> > + gfp_t alloc_gfp;
> > + unsigned int alloc_flags;
>
> Was going to complain that this is not set to ALLOC_WMARK_LOW. Must be faster
> next time...
>

Good that you caught it anyway!

> > + int allocated = 0;
> > +
> > + if (WARN_ON_ONCE(nr_pages <= 0))
> > + return 0;
> > +
> > + /* Use the single page allocator for one page. */
> > + if (nr_pages == 1)
> > + goto failed;
> > +
> > + /* May set ALLOC_NOFRAGMENT, fragmentation will return 1 page. */
>
> I don't understand this comment. Only alloc_flags_nofragment() sets this flag
> and we don't use it here?
>

It's there as a reminder that there are non-obvious consequences
to ALLOC_NOFRAGMENT that may affect the bulk allocation success
rate. __rmqueue_fallback will only select pageblock_order pages and if that
fails, we fall into the slow path that allocates a single page. I didn't
deal with it because it was not obvious that it's even relevant but I bet
in 6 months time, I'll forget that ALLOC_NOFRAGMENT may affect success
rates without the comment. I'm waiting for a bug that can trivially trigger
a case with a meaningful workload where the success rate is poor enough to
affect latency before adding complexity. Ideally by then, the allocation
paths would be unified a bit better.

> > + gfp &= gfp_allowed_mask;
> > + alloc_gfp = gfp;
> > + if (!prepare_alloc_pages(gfp, 0, preferred_nid, nodemask, &ac, &alloc_gfp, &alloc_flags))
> > + return 0;
> > + gfp = alloc_gfp;
> > +
> > + /* Find an allowed local zone that meets the high watermark. */
>
> Should it say "low watermark"?
>

Yeah, that's leftover from an earlier prototype :(

--
Mel Gorman
SUSE Labs