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

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Message-Id: <581DD097.5060400@linux.vnet.ibm.com>
Date:   Sat, 5 Nov 2016 17:59:11 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Xishi Qiu <qiuxishi@...wei.com>,
        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>
Cc:     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 11/05/2016 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.