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SubjectRe: [RFC] memory_hotplug: Free pages as pageblock_order
On Wed 12-09-18 14:56:45, Arun KS wrote:
> When free pages are done with pageblock_order, time spend on
> coalescing pages by buddy allocator can be reduced. With
> section size of 256MB, hot add latency of a single section
> shows improvement from 50-60 ms to less than 1 ms, hence
> improving the hot add latency by 60%.

Where does the improvement come from? You are still doing the same
amount of work except that the number of callbacks is lower. Is this the
real source of 60% improvement?

>
> If this looks okey, I'll modify users of set_online_page_callback
> and resend clean patch.

[...]

> +static int generic_online_pages(struct page *page, unsigned int order);
> +static online_pages_callback_t online_pages_callback = generic_online_pages;
> +
> +static int generic_online_pages(struct page *page, unsigned int order)
> +{
> + unsigned long nr_pages = 1 << order;
> + struct page *p = page;
> + unsigned int loop;
> +
> + for (loop = 0 ; loop < nr_pages ; loop++, p++) {
> + __ClearPageReserved(p);
> + set_page_count(p, 0);
> + }
> + adjust_managed_page_count(page, nr_pages);
> + init_page_count(page);
> + __free_pages(page, order);
> +
> + return 0;
> +}
> +
> +static int online_pages_blocks(unsigned long start_pfn, unsigned long nr_pages)
> +{
> + unsigned long pages_per_block = (1 << pageblock_order);
> + unsigned long nr_pageblocks = nr_pages / pages_per_block;
> +// unsigned long rem_pages = nr_pages % pages_per_block;
> + int i, ret, onlined_pages = 0;
> + struct page *page;
> +
> + for (i = 0 ; i < nr_pageblocks ; i++) {
> + page = pfn_to_page(start_pfn + (i * pages_per_block));
> + ret = (*online_pages_callback)(page, pageblock_order);
> + if (!ret)
> + onlined_pages += pages_per_block;
> + else if (ret > 0)
> + onlined_pages += ret;
> + }

Could you explain why does the pages_per_block step makes any sense? Why
don't you simply apply handle the full nr_pages worth of memory range
instead?

> +/*
> + if (rem_pages)
> + onlined_pages += online_page_single(start_pfn + i, rem_pages);
> +*/
> +
> + return onlined_pages;
> +}
> +
> static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
> void *arg)
> {
> - unsigned long i;
> unsigned long onlined_pages = *(unsigned long *)arg;
> - struct page *page;
>
> if (PageReserved(pfn_to_page(start_pfn)))
> - for (i = 0; i < nr_pages; i++) {
> - page = pfn_to_page(start_pfn + i);
> - (*online_page_callback)(page);
> - onlined_pages++;
> - }
> + onlined_pages = online_pages_blocks(start_pfn, nr_pages);
>
> online_mem_sections(start_pfn, start_pfn + nr_pages);
>
> --
> 1.9.1
>

--
Michal Hocko
SUSE Labs

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