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

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Date:   Mon, 7 Nov 2016 09:48:03 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.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 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?

Thanks,
Xishi Qiu

> 
> .
>