Re: [Xen-devel] [PATCH v5 1/2] memory_hotplug: Free pages as higher order

[Arun KS <arunks@xxxxxxxxxxxxxx>]

On 2018-10-10 21:00, Vlastimil Babka wrote:
> On 10/5/18 10:10 AM, Arun KS wrote:
> > When free pages are done with higher 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%. Modify external
> > providers of online callback to align with the change.
> >
> > Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>
> [...]
>
> > @@ -655,26 +655,44 @@ void __online_page_free(struct page *page)
> >  }
> >  EXPORT_SYMBOL_GPL(__online_page_free);
> >
> > -static void generic_online_page(struct page *page)
> > +static int generic_online_page(struct page *page, unsigned int order)
> >  {
> > -       __online_page_set_limits(page);
> This is now not called anymore, although the xen/hv variants still do
> it. The function seems empty these days, maybe remove it as a followup
> cleanup?
>
> > -       __online_page_increment_counters(page);
> > -       __online_page_free(page);
> > +       __free_pages_core(page, order);
> > +       totalram_pages += (1UL << order);
> > +#ifdef CONFIG_HIGHMEM
> > +       if (PageHighMem(page))
> > +               totalhigh_pages += (1UL << order);
> > +#endif
> __online_page_increment_counters() would have used
> adjust_managed_page_count() which would do the changes under
> managed_page_count_lock. Are we safe without the lock? If yes, there
> should perhaps be a comment explaining why.
Looks unsafe without managed_page_count_lock. I think better have a 
similar implementation of free_boot_core() in memory_hotplug.c like we 
had in version 1 of patch. And use adjust_managed_page_count() instead 
of page_zone(page)->managed_pages += nr_pages;
https://lore.kernel.org/patchwork/patch/989445/

-static void generic_online_page(struct page *page)
+static int generic_online_page(struct page *page, unsigned int order)
 {
-       __online_page_set_limits(page);
-       __online_page_increment_counters(page);
-       __online_page_free(page);
+       unsigned long nr_pages = 1 << order;
+       struct page *p = page;
+
+       for (loop = 0 ; loop < nr_pages ; loop++, p++) {
+               __ClearPageReserved(p);
+               set_page_count(p, 0);
+       }
+
+       adjust_managed_page_count(page, nr_pages);
+       set_page_refcounted(page);
+       __free_pages(page, order);
+
+       return 0;
+}


Regards,
Arun


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