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SubjectRe: [PATCH v2 3/5] mm/page_alloc: Optimize free_area_init_core
On Thu, Jul 19, 2018 at 10:52:35PM +0200, Oscar Salvador wrote:
> On Thu, Jul 19, 2018 at 05:15:55PM +0200, Michal Hocko wrote:
> > Your changelog doesn't really explain the motivation. Does the change
> > help performance? Is this a pure cleanup?
>
> Hi Michal,
>
> Sorry to not have explained this better from the very beginning.
>
> It should help a bit in performance terms as we would be skipping those
> condition checks and assignations for zones that do not have any pages.
> It is not a huge win, but I think that skipping code we do not really need to run
> is worh to have.
>
> > The function is certainly not an example of beauty. It is more an
> > example of changes done on top of older ones without much thinking. But
> > I do not see your change would make it so much better. I would consider
> > it a much nicer cleanup if it was split into logical units each doing
> > one specific thing.
>
> About the cleanup, I thought that moving that block of code to a separate function
> would make the code easier to follow.
> If you think that this is still not enough, I can try to split it and see the outcome.

I tried to split it innto three logical blocks:

- Substract memmap pages
- Substract dma reserves
- Account kernel pages (nr_kernel_pages and nr_total_pages)

Is this something that makes sense to you:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 10b754fba5fa..1397dcdd4a3c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6237,6 +6237,47 @@ static void pgdat_init_kcompactd(struct pglist_data *pgdat)
static void pgdat_init_kcompactd(struct pglist_data *pgdat) {}
#endif

+static void account_kernel_pages(enum zone_type j, unsigned long freesize,
+ unsigned long memmap_pages)
+{
+ if (!is_highmem_idx(j))
+ nr_kernel_pages += freesize;
+ /* Charge for highmem memmap if there are enough kernel pages */
+ else if (nr_kernel_pages > memmap_pages * 2)
+ nr_kernel_pages -= memmap_pages;
+ nr_all_pages += freesize;
+}
+
+static unsigned long substract_dma_reserves(unsigned long freesize)
+{
+ /* Account for reserved pages */
+ if (freesize > dma_reserve) {
+ freesize -= dma_reserve;
+ printk(KERN_DEBUG " %s zone: %lu pages reserved\n",
+ zone_names[0], dma_reserve);
+ }
+
+ return freesize;
+}
+
+static unsigned long substract_memmap_pages(unsigned long freesize, unsigned long memmap_pages)
+{
+ /*
+ * Adjust freesize so that it accounts for how much memory
+ * is used by this zone for memmap. This affects the watermark
+ * and per-cpu initialisations
+ */
+ if (freesize >= memmap_pages) {
+ freesize -= memmap_pages;
+ if (memmap_pages)
+ printk(KERN_DEBUG " %s zone: %lu pages used for memmap\n",
+ zone_names[j], memmap_pages);
+ } else
+ pr_warn(" %s zone: %lu pages exceeds freesize %lu\n",
+ zone_names[j], memmap_pages, freesize);
+ return freesize;
+}
+
/*
* Set up the zone data structures:
* - mark all pages reserved
@@ -6267,44 +6308,20 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat)

for (j = 0; j < MAX_NR_ZONES; j++) {
struct zone *zone = pgdat->node_zones + j;
- unsigned long size, freesize, memmap_pages;
+ unsigned long size = zone->spanned_pages
+ unsigned long freesize = zone->present_pages;
unsigned long zone_start_pfn = zone->zone_start_pfn;

- size = zone->spanned_pages;
- freesize = zone->present_pages;
-
- /*
- * Adjust freesize so that it accounts for how much memory
- * is used by this zone for memmap. This affects the watermark
- * and per-cpu initialisations
- */
- memmap_pages = calc_memmap_size(size, freesize);
- if (!is_highmem_idx(j)) {
- if (freesize >= memmap_pages) {
- freesize -= memmap_pages;
- if (memmap_pages)
- printk(KERN_DEBUG
- " %s zone: %lu pages used for memmap\n",
- zone_names[j], memmap_pages);
- } else
- pr_warn(" %s zone: %lu pages exceeds freesize %lu\n",
- zone_names[j], memmap_pages, freesize);
- }
+ if (size) {
+ unsigned long memmap_pages = calc_memmap_size(size, freesize);
+ if (!is_highmem_idx(j))
+ freesize = substract_memmap_pages(freesize, memmap_pages);

- /* Account for reserved pages */
- if (j == 0 && freesize > dma_reserve) {
- freesize -= dma_reserve;
- printk(KERN_DEBUG " %s zone: %lu pages reserved\n",
- zone_names[0], dma_reserve);
+ if (j == ZONE_DMA)
+ freesize = substract_dma_reserves(freesize);
+ account_kernel_pages(j, freesize, memmap_pages);
}

- if (!is_highmem_idx(j))
- nr_kernel_pages += freesize;
- /* Charge for highmem memmap if there are enough kernel pages */
- else if (nr_kernel_pages > memmap_pages * 2)
- nr_kernel_pages -= memmap_pages;
- nr_all_pages += freesize;

Thanks
--
Oscar Salvador
SUSE L3

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