linux-kernel - Re: [RFC][PATCH] mm: merge as soon as possible when pcp alloc/free

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Date:   Mon, 7 Nov 2016 10:20:50 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Xishi Qiu <qiuxishi@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "'Kirill A . Shutemov'" <kirill.shutemov@...ux.intel.com>,
        Taku Izumi <izumi.taku@...fujitsu.com>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] mm: merge as soon as possible when pcp alloc/free

On 11/07/2016 07:18 AM, Xishi Qiu wrote:
> On 2016/11/5 20:29, Anshuman Khandual wrote:
> 
>> On 11/05/2016 01:27 PM, Xishi Qiu wrote:
>>> Usually the memory of android phones is very small, so after a long
>>> running, the fragment is very large. Kernel stack which called by
>>> alloc_thread_stack_node() usually alloc 16K memory, and it failed
>>> frequently.
>>>
>>> However we have CONFIG_VMAP_STACK now, but it do not support arm64,
>>> and maybe it has some regression because of vmalloc, it need to
>>> find an area and create page table dynamically, this will take a short
>>> time.
>>>
>>> I think we can merge as soon as possible when pcp alloc/free to reduce
>>> fragment. The pcp page is hot page, so free it will cause cache miss,
>>> I use perf to test it, but it seems the regression is not so much, maybe
>>> it need to test more. Any reply is welcome.
>>
>> The idea of PCP is to have a fast allocation mechanism which does not depend
>> on an interrupt safe spin lock for every allocation. I am not very familiar
>> with this part of code but the following documentation from Mel Gorman kind
>> of explains that the this type of fragmentation problem which you might be
>> observing as one of the limitations of PCP mechanism.
>>
>> https://www.kernel.org/doc/gorman/html/understand/understand009.html
>> "Per CPU page list" sub header.
>>
> 
> "The last potential problem is that buddies of newly freed pages could exist
> in other pagesets leading to possible fragmentation problems."
> So we should not change it, and this is a known issue, right?

Seems like that.