Re: [PATCH v1 4/5] mm/page_alloc: place pages to tail in __free_pages_core()

From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@suse.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-hyperv@vger.kernel.org, xen-devel@lists.xenproject.org,
	linux-acpi@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Oscar Salvador <osalvador@suse.de>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Dave Hansen <dave.hansen@intel.com>,
	Wei Yang <richard.weiyang@linux.alibaba.com>,
	Mike Rapoport <rppt@kernel.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>
Subject: Re: [PATCH v1 4/5] mm/page_alloc: place pages to tail in __free_pages_core()
Date: Fri, 2 Oct 2020 17:10:03 +0200	[thread overview]
Message-ID: <7bf5d426-1fdd-4525-25da-a68d92b6c11d@redhat.com> (raw)
In-Reply-To: <20201002134115.GJ4555@dhcp22.suse.cz>

On 02.10.20 15:41, Michal Hocko wrote:
> On Mon 28-09-20 20:21:09, David Hildenbrand wrote:
>> __free_pages_core() is used when exposing fresh memory to the buddy
>> during system boot and when onlining memory in generic_online_page().
>>
>> generic_online_page() is used in two cases:
>>
>> 1. Direct memory onlining in online_pages().
>> 2. Deferred memory onlining in memory-ballooning-like mechanisms (HyperV
>>    balloon and virtio-mem), when parts of a section are kept
>>    fake-offline to be fake-onlined later on.
>>
>> In 1, we already place pages to the tail of the freelist. Pages will be
>> freed to MIGRATE_ISOLATE lists first and moved to the tail of the freelists
>> via undo_isolate_page_range().
>>
>> In 2, we currently don't implement a proper rule. In case of virtio-mem,
>> where we currently always online MAX_ORDER - 1 pages, the pages will be
>> placed to the HEAD of the freelist - undesireable. While the hyper-v
>> balloon calls generic_online_page() with single pages, usually it will
>> call it on successive single pages in a larger block.
>>
>> The pages are fresh, so place them to the tail of the freelists and avoid
>> the PCP. In __free_pages_core(), remove the now superflouos call to
>> set_page_refcounted() and add a comment regarding page initialization and
>> the refcount.
>>
>> Note: In 2. we currently don't shuffle. If ever relevant (page shuffling
>> is usually of limited use in virtualized environments), we might want to
>> shuffle after a sequence of generic_online_page() calls in the
>> relevant callers.
> 
> It took some time to get through all the freeing paths with subtle
> differences but this looks reasonable. You are mentioning that this
> influences a boot time free memory ordering as well but only very
> briefly. I do not expect this to make a huge difference but who knows.
> It makes some sense to add pages in the order they show up in the
> physical address ordering.

I think boot memory is mostly exposed in the physical address ordering.
In that case, higher addresses will now be used less likely immediately
after this patch. I also don't think it's an issue - if we still detect
it's an issue it's fairly easy to change again.

Thanks!

-- 
Thanks,

David / dhildenb