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

[Michal Hocko <mhocko@xxxxxxxxxx>]

On Thu 11-10-18 10:07:02, Vlastimil Babka wrote:
> On 10/10/18 6:56 PM, Arun KS wrote:
> > 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/
> 
> Looks like deferred_free_range() has the same problem calling
> __free_pages_core() to adjust zone->managed_pages.

deferred initialization has one thread per node AFAIR so we cannot race
on managed_pages updates. Well, unless some of the mentioned can run
that early which I dunno.

> __free_pages_bootmem() is OK because at that point the system is still
> single-threaded?
> Could be solved by moving that out of __free_pages_core().
> 
> But do we care about readers potentially seeing a store tear? If yes
> then maybe these counters should be converted to atomics...

I wanted to suggest that already but I have no idea whether the lock
instructions would cause more overhead.
-- 
Michal Hocko
SUSE Labs

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