Re: [PATCH 01/10] mm: control memory placement by nodemask for two tier main memory

[Dan Williams]

On Mon, Mar 25, 2019 at 12:28 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 3/23/19 10:21 AM, Dan Williams wrote:
> > On Fri, Mar 22, 2019 at 9:45 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> >> ---
> >>   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.

> 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.