linux-kernel - Re: [PATCH 3/3] mm: Enable Buddy allocation isolation for CDM nodes

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Message-Id: <81ae434a-0c67-c11a-e052-8b33b39c2152@linux.vnet.ibm.com>
Date:   Thu, 9 Feb 2017 15:39:58 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     mhocko@...e.com, mgorman@...e.de, minchan@...nel.org,
        aneesh.kumar@...ux.vnet.ibm.com, bsingharora@...il.com,
        srikar@...ux.vnet.ibm.com, haren@...ux.vnet.ibm.com,
        jglisse@...hat.com, dave.hansen@...el.com, dan.j.williams@...el.com
Subject: Re: [PATCH 3/3] mm: Enable Buddy allocation isolation for CDM nodes

On 02/09/2017 02:18 PM, Vlastimil Babka wrote:
> On 02/09/2017 06:05 AM, Anshuman Khandual wrote:
>> On 02/08/2017 10:48 PM, Vlastimil Babka wrote:
>>> On 02/08/2017 03:01 PM, Anshuman Khandual wrote:
>>>> This implements allocation isolation for CDM nodes in buddy
>>>> allocator by
>>>> discarding CDM memory zones all the time except in the cases where the
>>>> gfp
>>>> flag has got __GFP_THISNODE or the nodemask contains CDM nodes in cases
>>>> where it is non NULL (explicit allocation request in the kernel or user
>>>> process MPOL_BIND policy based requests).
>>>>
>>>> Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
>>>> ---
>>>>  mm/page_alloc.c | 19 +++++++++++++++++++
>>>>  1 file changed, 19 insertions(+)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 40908de..7d8c82a 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -64,6 +64,7 @@
>>>>  #include <linux/page_owner.h>
>>>>  #include <linux/kthread.h>
>>>>  #include <linux/memcontrol.h>
>>>> +#include <linux/node.h>
>>>>
>>>>  #include <asm/sections.h>
>>>>  #include <asm/tlbflush.h>
>>>> @@ -2908,6 +2909,24 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned
>>>> int order, int alloc_flags,
>>>>          struct page *page;
>>>>          unsigned long mark;
>>>>
>>>> +        /*
>>>> +         * CDM nodes get skipped if the requested gfp flag
>>>> +         * does not have __GFP_THISNODE set or the nodemask
>>>> +         * does not have any CDM nodes in case the nodemask
>>>> +         * is non NULL (explicit allocation requests from
>>>> +         * kernel or user process MPOL_BIND policy which has
>>>> +         * CDM nodes).
>>>> +         */
>>>> +        if (is_cdm_node(zone->zone_pgdat->node_id)) {
>>>> +            if (!(gfp_mask & __GFP_THISNODE)) {
>>>> +                if (!ac->nodemask)
>>>> +                    continue;
>>>> +
>>>> +                if (!nodemask_has_cdm(*ac->nodemask))
>>>> +                    continue;
>>>
>>> nodemask_has_cdm() looks quite expensive, combined with the loop here
>>> that's O(n^2). But I don't understand why you need it. If there is no
>>> cdm node in the nodemask, then we never reach this code with a cdm node,
>>> because the zonelist iterator already checks the nodemask? Am I missing
>>> something?
>>
>> A CDM zone can be selected during zonelist iteration if
>>
>>     (1) If nodemask is NULL (where all zones are eligible)
>>
>>         (1) Skip it if __GFP_THISNODE is not mentioned
>>         (2) Pick it if __GFP_THISNODE is mentioned
>>
>>     (2) If nodemask has CDM (where CDM zones are eligible)
>>
>>         (1) Pick it if nodemask has CDM
>>         (2) Pick it if __GFP_THISNODE is mentioned
>>
>> (1) (1) Enforces the primary isolation
>> (2) (1) Is the only option which could be O(n^2) as the worst case
>>
>> Checking for both the zone being a CDM zone and the nodemask containing
>> CDM node has to happen together for (2) (1). But we dont run into this
>> option unless we have first checked if request contains __GFP_THISNODE
>> and that nodemask is really a non NULL value. Hence the number cases
>> getting into (2) (1) should be less. IIUC only the user space MPOL_BIND
>> ones will come here.
> 
> Maybe I'm still missing something, but when you do nodemask_has_cdm()
> above then we already passed "if (!ac->nodemask) continue" which means
> ac->nodemask is not null, which means the zonelist iterator already did
> the filtering on ac->nodemask, and if this zone passed the filter and
> it's a cdm zone, then it has to be set in the nodemask?

Hmm, think you are right. Then I can drop the last check there. Will test
it out. Thanks for pointing this out.