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

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

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@linux.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.