[RFC] autonuma: Migrate on fault among multiple bound nodes

Huang Ying
2020-09-16T01:00:15Z

Now, AutoNUMA can only optimize the page placement among the NUMA
nodes if the default memory policy is used. Because the memory policy
specified explicitly should take precedence. But this seems too
strict in some situations. For example, on a system with 4 NUMA
nodes, if the memory of an application is bound to the node 0 and 1,
AutoNUMA can potentially migrate the pages between the node 0 and 1 to
reduce cross-node accessing without breaking the explicit memory
binding policy.

So in this patch, if MPOL_BIND is used to bind the memory of the
application to multiple nodes, and in the hint page fault handler both
the faulting page node and the accessing node are in the policy
nodemask, the page will be tried to be migrated to the accessing node
to reduce the cross-node accessing.

Questions:

Sysctl knob kernel.numa_balancing can enable/disable AutoNUMA
optimizing globally. And now, it appears that the explicit NUMA
memory policy specifying (e.g. via numactl, mbind(), etc.) acts like
an implicit per-thread/VMA knob to enable/disable the AutoNUMA
optimizing for the thread/VMA. Although this looks like a side effect
instead of an API, from commit fc3147245d19 ("mm: numa: Limit NUMA
scanning to migrate-on-fault VMAs"), this is used by some users? So
the question is, do we need an explicit per-thread/VMA knob to
enable/disable AutoNUMA optimizing for the thread/VMA? Or just use
the global knob, either optimize all thread/VMAs as long as the
explicitly specified memory policies are respected, or don't optimize
at all.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
---
mm/mempolicy.c | 43 +++++++++++++++++++++++++++++++------------
1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index eddbe4e56c73..a941eab2de24 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1827,6 +1827,13 @@ static struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
return pol;
}

+static bool mpol_may_mof(struct mempolicy *pol)
+{
+ /* May migrate among bound nodes for MPOL_BIND */
+ return pol->flags & MPOL_F_MOF ||
+ (pol->mode == MPOL_BIND && nodes_weight(pol->v.nodes) > 1);
+}
+
bool vma_policy_mof(struct vm_area_struct *vma)
{
struct mempolicy *pol;
@@ -1835,7 +1842,7 @@ bool vma_policy_mof(struct vm_area_struct *vma)
bool ret = false;

pol = vma->vm_ops->get_policy(vma, vma->vm_start);
- if (pol && (pol->flags & MPOL_F_MOF))
+ if (pol && mpol_may_mof(pol))
ret = true;
mpol_cond_put(pol);

@@ -1846,7 +1853,7 @@ bool vma_policy_mof(struct vm_area_struct *vma)
if (!pol)
pol = get_task_policy(current);

- return pol->flags & MPOL_F_MOF;
+ return mpol_may_mof(pol);
}

static int apply_policy_zone(struct mempolicy *policy, enum zone_type zone)
@@ -2474,11 +2481,13 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
int thisnid = cpu_to_node(thiscpu);
int polnid = NUMA_NO_NODE;
int ret = -1;
+ bool moron;

pol = get_vma_policy(vma, addr);
- if (!(pol->flags & MPOL_F_MOF))
+ if (!mpol_may_mof(pol))
goto out;

+ moron = pol->flags & MPOL_F_MORON;
switch (pol->mode) {
case MPOL_INTERLEAVE:
pgoff = vma->vm_pgoff;
@@ -2494,20 +2503,30 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
break;

case MPOL_BIND:
-
/*
- * allows binding to multiple nodes.
- * use current page if in policy nodemask,
- * else select nearest allowed node, if any.
- * If no allowed nodes, use current [!misplaced].
+ * Allows binding to multiple nodes. If both current and
+ * accessing nodes are in policy nodemask, migrate to
+ * accessing node to optimize page placement. Otherwise,
+ * use current page if in policy nodemask or MPOL_F_MOF not
+ * set, else select nearest allowed node, if any. If no
+ * allowed nodes, use current [!misplaced].
*/
- if (node_isset(curnid, pol->v.nodes))
+ if (node_isset(curnid, pol->v.nodes)) {
+ if (node_isset(thisnid, pol->v.nodes)) {
+ moron = true;
+ polnid = thisnid;
+ } else {
+ goto out;
+ }
+ } else if (!(pol->flags & MPOL_F_MOF)) {
goto out;
- z = first_zones_zonelist(
+ } else {
+ z = first_zones_zonelist(
node_zonelist(numa_node_id(), GFP_HIGHUSER),
gfp_zone(GFP_HIGHUSER),
&pol->v.nodes);
- polnid = zone_to_nid(z->zone);
+ polnid = zone_to_nid(z->zone);
+ }
break;

default:
@@ -2515,7 +2534,7 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
}

/* Migrate the page towards the node whose CPU is referencing it */
- if (pol->flags & MPOL_F_MORON) {
+ if (moron) {
polnid = thisnid;

if (!should_numa_migrate_memory(current, page, curnid, thiscpu))
--
2.28.0