On Sun, 14 Mar 2021 12:52:32 +0000 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote: > mm/page_alloc: Add an array-based interface to the bulk page allocator > > The existing callers for the bulk allocator are storing the pages in > arrays. This patch adds an array-based interface to the API to avoid > multiple list iterations. The page list interface is preserved to > avoid requiring all users of the bulk API to allocate and manage > enough storage to store the pages. I'm testing this patch, see results below and in commit[1]. The array variant is clearly faster in these micro-benchmarks. (Some comment inlined about code) The change to my page_bench04_bulk is here[1]: [1] https://github.com/netoptimizer/prototype-kernel/commit/4c41fe0d4107f514 Notice these "per elem" measurements are alloc+free cost for order-0 pages. BASELINE single_page alloc+put: 207 cycles(tsc) 57.773 ns LIST variant: time_bulk_page_alloc_free_list: step=bulk size Per elem: 294 cycles(tsc) 81.866 ns (step:1) Per elem: 214 cycles(tsc) 59.477 ns (step:2) Per elem: 199 cycles(tsc) 55.504 ns (step:3) Per elem: 192 cycles(tsc) 53.489 ns (step:4) Per elem: 188 cycles(tsc) 52.456 ns (step:8) Per elem: 184 cycles(tsc) 51.346 ns (step:16) Per elem: 183 cycles(tsc) 50.883 ns (step:32) Per elem: 184 cycles(tsc) 51.236 ns (step:64) Per elem: 189 cycles(tsc) 52.620 ns (step:128) ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size Per elem: 195 cycles(tsc) 54.174 ns (step:1) Per elem: 123 cycles(tsc) 34.224 ns (step:2) Per elem: 113 cycles(tsc) 31.430 ns (step:3) Per elem: 108 cycles(tsc) 30.003 ns (step:4) Per elem: 102 cycles(tsc) 28.417 ns (step:8) Per elem: 98 cycles(tsc) 27.475 ns (step:16) Per elem: 96 cycles(tsc) 26.901 ns (step:32) Per elem: 95 cycles(tsc) 26.487 ns (step:64) Per elem: 94 cycles(tsc) 26.170 ns (step:128) The array variant is clearly faster. It it worth mentioning that bulk=1 result in fallback to normal single page allocation via __alloc_pages(). > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h > index 4a304fd39916..fb6234e1fe59 100644 > --- a/include/linux/gfp.h > +++ b/include/linux/gfp.h > @@ -520,13 +520,20 @@ struct page *__alloc_pages(gfp_t gfp, unsigned int order, int preferred_nid, > > int __alloc_pages_bulk(gfp_t gfp, int preferred_nid, > nodemask_t *nodemask, int nr_pages, > - struct list_head *list); > + struct list_head *page_list, > + struct page **page_array); > > /* Bulk allocate order-0 pages */ > static inline unsigned long > -alloc_pages_bulk(gfp_t gfp, unsigned long nr_pages, struct list_head *list) > +alloc_pages_bulk_list(gfp_t gfp, unsigned long nr_pages, struct list_head *list) > { > - return __alloc_pages_bulk(gfp, numa_mem_id(), NULL, nr_pages, list); > + return __alloc_pages_bulk(gfp, numa_mem_id(), NULL, nr_pages, list, NULL); > +} > + > +static inline unsigned long > +alloc_pages_bulk_array(gfp_t gfp, unsigned long nr_pages, struct page **page_array) > +{ > + return __alloc_pages_bulk(gfp, numa_mem_id(), NULL, nr_pages, NULL, page_array); > } > > /* > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 3e0c87c588d3..96590f0726c7 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4965,13 +4965,20 @@ static inline bool prepare_alloc_pages(gfp_t gfp, unsigned int order, > > /* > * This is a batched version of the page allocator that attempts to > - * allocate nr_pages quickly from the preferred zone and add them to list. > + * allocate nr_pages quickly from the preferred zone. Pages are added > + * to page_list if page_list is not NULL, otherwise it is assumed > + * that the page_array is valid. > + * > + * If using page_array, only NULL elements are populated with pages. > + * The caller must ensure that the array has enough NULL elements > + * to store nr_pages or the buffer overruns. > * > * Returns the number of pages allocated. > */ > int __alloc_pages_bulk(gfp_t gfp, int preferred_nid, > nodemask_t *nodemask, int nr_pages, > - struct list_head *alloc_list) > + struct list_head *page_list, > + struct page **page_array) > { > struct page *page; > unsigned long flags; > @@ -4987,6 +4994,9 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid, > if (WARN_ON_ONCE(nr_pages <= 0)) > return 0; > > + if (WARN_ON_ONCE(!page_list && !page_array)) > + return 0; > + > if (nr_pages == 1) > goto failed; > > @@ -5035,7 +5045,24 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid, > break; > } > > - list_add(&page->lru, alloc_list); > + if (page_list) { > + /* New page prep is deferred */ > + list_add(&page->lru, page_list); > + } else { > + /* Skip populated elements */ > + while (*page_array) > + page_array++; I don't like this approach as it is a dangerous construct, that can run wild through the memory. I have coded up another approach where I have an nr_avail counter instead, that will "include" and count existing elements in the array. > + > + /* > + * Array pages must be prepped immediately to > + * avoid tracking which pages are new and > + * which ones were already on the array. > + */ > + prep_new_page(page, 0, gfp, 0); > + *page_array = page; > + page_array++; > + } > + > allocated++; > } > > @@ -5044,9 +5071,12 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid, > > local_irq_restore(flags); > > - /* Prep page with IRQs enabled to reduce disabled times */ > - list_for_each_entry(page, alloc_list, lru) > - prep_new_page(page, 0, gfp, 0); > + /* Prep pages with IRQs enabled if using a list */ > + if (page_list) { > + list_for_each_entry(page, page_list, lru) { > + prep_new_page(page, 0, gfp, 0); > + } > + } > > return allocated; > > @@ -5056,7 +5086,10 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid, > failed: > page = __alloc_pages(gfp, 0, preferred_nid, nodemask); > if (page) { > - list_add(&page->lru, alloc_list); > + if (page_list) > + list_add(&page->lru, page_list); > + else > + *page_array = page; > allocated = 1; > } > -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
