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

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Date:   Wed, 10 Oct 2018 22:26:41 +0530
From:   Arun KS <arunks@...eaurora.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        mhocko@...e.com, iamjoonsoo.kim@....com,
        gregkh@...uxfoundation.org, osalvador@...e.de, malat@...ian.org,
        kirill.shutemov@...ux.intel.com, jrdr.linux@...il.com,
        yasu.isimatu@...il.com, mgorman@...hsingularity.net,
        aaron.lu@...el.com, devel@...uxdriverproject.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, vatsa@...eaurora.org,
        vinmenon@...eaurora.org, getarunks@...il.com
Subject: Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

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@...eaurora.org>
> 
> [...]
> 
>> @@ -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