Re: [PATCH v3 2/2] mm/numa_balancing:Allow migrate on protnone reference with MPOL_PREFERRED_MANY policy
From: Donet Tom
Date: Mon Mar 25 2024 - 08:54:35 EST
On 3/25/24 08:18, Huang, Ying wrote:
Donet Tom <donettom@xxxxxxxxxxxxx> writes:
On 3/22/24 14:02, Huang, Ying wrote:
Donet Tom <donettom@xxxxxxxxxxxxx> writes:
commit bda420b98505 ("numa balancing: migrate on fault among multiple bound
nodes") added support for migrate on protnone reference with MPOL_BIND
memory policy. This allowed numa fault migration when the executing node
is part of the policy mask for MPOL_BIND. This patch extends migration
support to MPOL_PREFERRED_MANY policy.
Currently, we cannot specify MPOL_PREFERRED_MANY with the mempolicy flag
MPOL_F_NUMA_BALANCING. This causes issues when we want to use
NUMA_BALANCING_MEMORY_TIERING. To effectively use the slow memory tier,
the kernel should not allocate pages from the slower memory tier via
allocation control zonelist fallback. Instead, we should move cold pages
from the faster memory node via memory demotion. For a page allocation,
kswapd is only woken up after we try to allocate pages from all nodes in
the allocation zone list. This implies that, without using memory
policies, we will end up allocating hot pages in the slower memory tier.
MPOL_PREFERRED_MANY was added by commit b27abaccf8e8 ("mm/mempolicy: add
MPOL_PREFERRED_MANY for multiple preferred nodes") to allow better
allocation control when we have memory tiers in the system. With
MPOL_PREFERRED_MANY, the user can use a policy node mask consisting only
of faster memory nodes. When we fail to allocate pages from the faster
memory node, kswapd would be woken up, allowing demotion of cold pages
to slower memory nodes.
With the current kernel, such usage of memory policies implies we can't
do page promotion from a slower memory tier to a faster memory tier
using numa fault. This patch fixes this issue.
For MPOL_PREFERRED_MANY, if the executing node is in the policy node
mask, we allow numa migration to the executing nodes. If the executing
node is not in the policy node mask, we do not allow numa migration.
Can we provide more information about this? I suggest to use an
example, for instance, pages may be distributed among multiple sockets
unexpectedly.
Thank you for your suggestion. However, this commit message explains all the scenarios.
Yes. The commit message is correct and covers many cases. What I
suggested is to describe why we do that? An examples can not covers all
possibility, but it is easy to be understood. For example, something as
below?
For example, on a 2-sockets system, there are N0, N1, N2 in socket 0, N3
in socket 1. N0, N1, N3 have fast memory and CPU, while N2 has slow
memory and no CPU. For a workload, we may use MPOL_PREFERRED_MANY with
nodemask with N0 and N1 set because the workload runs on CPUs of socket
0 at most times. Then, even if the workload runs on CPUs of N3
occasionally, we will not try to migrate the workload pages from N2 to
N3 because users may want to avoid cross-socket access as much as
possible in the long term.
Thank you. I will change the commit message and post V4.
Thanks
Donet Tom
For example, Consider a system with 3 numa nodes (N0,N1 and N6).
N0 and N1 are tier1 DRAM nodes and N6 is tier 2 PMEM node.
Scenario 1: The process is executing on N1,
If the executing node is in the policy node mask,
Curr Loc Pages - The numa node where page present(folio node)
==================================================================================
Process Policy Curr Loc Pages Observations
-----------------------------------------------------------------------------------
N1 N0 N1 N6 N0 Pages Migrated from N0 to N1
N1 N0 N1 N6 N6 Pages Migrated from N6 to N1
N1 N0 N1 N1 Pages Migrated from N1 to N6
Pages are not Migrating ?
N1 N0 N1 N6 Pages Migrated from N6 to N1
------------------------------------------------------------------------------------
Scenario 2: The process is executing on N1,
If the executing node is NOT in the policy node mask,
Curr Loc Pages - The numa node where page present(folio node)
===================================================================================
Process Policy Curr Loc Pages Observations
-----------------------------------------------------------------------------------
N1 N0 N6 N0 Pages are not Migrating
N1 N0 N6 N6 Pages are not migration,
N1 N0 N0 Pages are not Migrating
------------------------------------------------------------------------------------
Scenario 3: The process is executing on N1,
If the executing node and folio nodes are NOT in the policy node mask,
Curr Loc Pages - The numa node where page present (folio node)
====================================================================================
Thread Policy Curr Loc Pages Observations
------------------------------------------------------------------------------------
N1 N0 N6 Pages are not Migrating
N1 N6 N0 Pages are not Migrating
------------------------------------------------------------------------------------
We can conclude that even if the pages are distributed among multiple sockets,
if the executing node is in the policy node mask, we allow numa migration to the
executing nodes. If the executing node is not in the policy node mask,
we do not allow numa migration.
[snip]
--
Best Regards,
Huang, Ying