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Subject: Re: [PATCH 01/10] mm: control memory placement by nodemask for two
 tier main memory
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Rik van Riel <riel@surriel.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Keith Busch <keith.busch@intel.com>,
        Fengguang Wu <fengguang.wu@intel.com>,
        "Du, Fan" <fan.du@intel.com>, "Huang, Ying" <ying.huang@intel.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Vishal L Verma <vishal.l.verma@intel.com>
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 <CAPcyv4g5RoHhXhkKQaYkqYLN1y3KavbGeM1zVus-3fY5Q+JdxA@mail.gmail.com>
 <688dffbc-2adc-005d-223e-fe488be8c5fc@linux.alibaba.com>
 <CAPcyv4g3xzuS8hP9jOX_BXWyFEH32YfCEDs3a_K_VRODfATc=Q@mail.gmail.com>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <406a78f6-9bac-b0f8-9acc-b72540a72a11@linux.alibaba.com>
Date:   Mon, 25 Mar 2019 16:36:10 -0700
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On 3/25/19 4:18 PM, Dan Williams wrote:
> On Mon, Mar 25, 2019 at 12:28 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>
>>
>> On 3/23/19 10:21 AM, Dan Williams wrote:
>>> On Fri, Mar 22, 2019 at 9:45 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>> When running applications on the machine with NVDIMM as NUMA node, the
>>>> memory allocation may end up on NVDIMM node.  This may result in silent
>>>> performance degradation and regression due to the difference of hardware
>>>> property.
>>>>
>>>> DRAM first should be obeyed to prevent from surprising regression.  Any
>>>> non-DRAM nodes should be excluded from default allocation.  Use nodemask
>>>> to control the memory placement.  Introduce def_alloc_nodemask which has
>>>> DRAM nodes set only.  Any non-DRAM allocation should be specified by
>>>> NUMA policy explicitly.
>>>>
>>>> In the future we may be able to extract the memory charasteristics from
>>>> HMAT or other source to build up the default allocation nodemask.
>>>> However, just distinguish DRAM and PMEM (non-DRAM) nodes by SRAT flag
>>>> for the time being.
>>>>
>>>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>>>> ---
>>>>    arch/x86/mm/numa.c     |  1 +
>>>>    drivers/acpi/numa.c    |  8 ++++++++
>>>>    include/linux/mmzone.h |  3 +++
>>>>    mm/page_alloc.c        | 18 ++++++++++++++++--
>>>>    4 files changed, 28 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
>>>> index dfb6c4d..d9e0ca4 100644
>>>> --- a/arch/x86/mm/numa.c
>>>> +++ b/arch/x86/mm/numa.c
>>>> @@ -626,6 +626,7 @@ static int __init numa_init(int (*init_func)(void))
>>>>           nodes_clear(numa_nodes_parsed);
>>>>           nodes_clear(node_possible_map);
>>>>           nodes_clear(node_online_map);
>>>> +       nodes_clear(def_alloc_nodemask);
>>>>           memset(&numa_meminfo, 0, sizeof(numa_meminfo));
>>>>           WARN_ON(memblock_set_node(0, ULLONG_MAX, &memblock.memory,
>>>>                                     MAX_NUMNODES));
>>>> diff --git a/drivers/acpi/numa.c b/drivers/acpi/numa.c
>>>> index 867f6e3..79dfedf 100644
>>>> --- a/drivers/acpi/numa.c
>>>> +++ b/drivers/acpi/numa.c
>>>> @@ -296,6 +296,14 @@ void __init acpi_numa_slit_init(struct acpi_table_slit *slit)
>>>>                   goto out_err_bad_srat;
>>>>           }
>>>>
>>>> +       /*
>>>> +        * Non volatile memory is excluded from zonelist by default.
>>>> +        * Only regular DRAM nodes are set in default allocation node
>>>> +        * mask.
>>>> +        */
>>>> +       if (!(ma->flags & ACPI_SRAT_MEM_NON_VOLATILE))
>>>> +               node_set(node, def_alloc_nodemask);
>>> Hmm, no, I don't think we should do this. Especially considering
>>> current generation NVDIMMs are energy backed DRAM there is no
>>> performance difference that should be assumed by the non-volatile
>>> flag.
>> Actually, here I would like to initialize a node mask for default
>> allocation. Memory allocation should not end up on any nodes excluded by
>> this node mask unless they are specified by mempolicy.
>>
>> We may have a few different ways or criteria to initialize the node
>> mask, for example, we can read from HMAT (when HMAT is ready in the
>> future), and we definitely could have non-DRAM nodes set if they have no
>> performance difference (I'm supposed you mean NVDIMM-F  or HBM).
>>
>> As long as there are different tiers, distinguished by performance, for
>> main memory, IMHO, there should be a defined default allocation node
>> mask to control the memory placement no matter where we get the information.
> I understand the intent, but I don't think the kernel should have such
> a hardline policy by default. However, it would be worthwhile
> mechanism and policy to consider for the dax-hotplug userspace
> tooling. I.e. arrange for a given device-dax instance to be onlined,
> but set the policy to require explicit opt-in by numa binding for it
> to be an allocation / migration option.
>
> I added Vishal to the cc who is looking into such policy tooling.

We may assume the nodes returned by cpu_to_node() would be treated as 
the default allocation nodes from the kernel point of view.

So, the below code may do the job:

diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
index d9e0ca4..a3e07da 100644
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c
@@ -764,6 +764,8 @@ void __init init_cpu_to_node(void)
                         init_memory_less_node(node);

                 numa_set_node(cpu, node);
+
+              node_set(node, def_alloc_nodemask);
         }
  }

Actually, the kernel should not care too much what kind of memory is 
used, any node could be used for memory allocation. But it may be better 
to restrict to some default nodes due to the performance disparity, for 
example, default to regular DRAM only. Here kernel assumes the nodes 
associated with CPUs would be DRAM nodes.

The node mask could be exported to user space to be override by 
userspace tool or sysfs or kernel commandline. But I still think kernel 
does need a default node mask.

>
>> But, for now we haven't had such information ready for such use yet, so
>> the SRAT flag might be a choice.
>>
>>> Why isn't default SLIT distance sufficient for ensuring a DRAM-first
>>> default policy?
>> "DRAM-first" may sound ambiguous, actually I mean "DRAM only by
>> default". SLIT should just can tell us what node is local what node is
>> remote, but can't tell us the performance difference.
> I think it's a useful semantic, but let's leave the selection of that
> policy to an explicit userspace decision.

Yes, mempolicy is a kind of userspace decision too.

Thanks,
Yang