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

[Vlastimil Babka <vbabka@xxxxxxx>]

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. I expect
__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...

> -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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