On Wed, Mar 30, 2022 at 07:21:23PM +0530, Srikar Dronamraju wrote:
> arch/powerpc/mm/numa.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index b9b7fefbb64b..13022d734951 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1436,7 +1436,7 @@ int find_and_online_cpu_nid(int cpu)
> if (new_nid < 0 || !node_possible(new_nid))
> new_nid = first_online_node;
>
> - if (NODE_DATA(new_nid) == NULL) {
> + if (!node_online(new_nid)) {
> #ifdef CONFIG_MEMORY_HOTPLUG
> /*
> * Need to ensure that NODE_DATA is initialized for a node from
Because of this fix, I wanted to check whether we might have any more fallouts due
to ("mm: handle uninitialized numa nodes gracefully"), and it made me look closer
as to why powerpc is the only platform that special cases try_online_node(),
while all others rely on cpu_up()->try_online_node() to do the right thing.
So, I had a look.
Let us rewind a bit:
The commit that sets find_and_online_cpu_nid() in dlpar_online_cpu was
e67e02a544e9 ("powerpc/pseries: Fix cpu hotplug crash with memoryless nodes").
In there, it says that we have the following picture:
partition_sched_domains
arch_update_cpu_topology
numa_update_cpu_topology
find_and_online_cpu_nid
and by the time find_and_online_cpu_nid() gets called to online the node, it might be
too late as we might have referenced some NODE_DATA() fields.
Note that this happens at a much later stage in cpuhp.
Also note that at a much earlier stage, we do already have a try_online_node() in cpu_up(),
which should allocate-and-online the node and prevent accessing garbage.
But the problem is that, on powerpc, all possible cpus have the same node set at boot stage
(see arch/powerpc/mm/numa.c:mem_topology_setup),
so cpu_to_node() returns the same thing until it the mapping gets update (which happens in
start_secondary()->set_numa_node()), and so, the try_online_node() from cpu_up() does not
apply on the right node, because it still holds the not-up-to-date mapping node <-> cpu.
(e.g: in my test case, when onlining a CPU belongin to node1, cpu_up()->try_online_node()
tries to online node0, or whatever old mapping numa<->cpu is there).
So, we have something like:
dlpar_online_cpu
device_online
dev->bus->online
cpu_subsys_online
cpu_device_up
cpu_up
try_online_node (old mapping nid <-> cpu )
...
...
cphp_callbacks
sched_cpu_activate
cpuset_update_active_cpus
schedule_work(&cpuset_hotplug_work)
cpuset_hotplug_work
partition_sched_domains
arch_update_cpu_topology
numa_update_cpu_topology
find_and_online_cpu_nid (online new_nid)
So, yeah, the real onlining in numa_update_cpu_topology()->find_and_online_cpu_nid()
happens too late, that is why adding find_and_online_cpu_nid() back in dlpar_online_cpu()
fixed the issue, but we should not need this special casing at all.
We do already know the numa<->cpu associativity in
dlpar_online_cpu()->find_and_online_cpu_nid(), so we should just be able to
update the numa<->cpu mapping, and let the try_online_node() do the right thing.
With this in mind, I came up with the following patch, where I carried out a battery
of tests for CPU hotplug stuff and it worked as expected.
But I am not familiar with all powerpc pitfalls, e.g: dedicated vs shared cpus etc, so
I would like to hear from more familiar people.
The patch is:
From: Oscar Salvador <osalvador@xxxxxxx>
Date: Wed, 6 Apr 2022 14:39:15 +0200
Subject: [PATCH] powerpc/numa: Associate numa node to its cpu earlier
powerpc is the only platform that do not rely on
cpu_up()->try_online_node() to bring up a numa node,
and special cases it, instead, deep in its own machinery:
dlpar_online_cpu
find_and_online_cpu_nid
try_online_node
This should not be needed, but the thing is that the try_online_node()
from cpu_up() will not apply on the right node, because cpu_to_node()
will return the old mapping numa<->cpu that gets set on boot stage
for all possible cpus.
That can be seen easily if we try to print out the numa node passed
to try_online_node() in cpu_up().
The thing is that the numa<->cpu mapping does not get updated till a much
later stage in start_secondary:
start_secondary:
set_numa_node(numa_cpu_lookup_table[cpu])
But we do not really care, as we already now the
CPU <-> NUMA associativity back in find_and_online_cpu_nid(),
so let us make use of that and set the proper numa<->cpu mapping,
so cpu_to_node() in cpu_up() returns the right node and
try_online_node() can do its work.
Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
arch/powerpc/include/asm/topology.h | 8 ++-----
arch/powerpc/mm/numa.c | 31 +++++++---------------------
arch/powerpc/platforms/pseries/hotplug-cpu.c | 2 +-
3 files changed, 11 insertions(+), 30 deletions(-)
diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index 36fcafb1fd6d..6ae1b2dce83e 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -111,14 +111,10 @@ static inline void unmap_cpu_from_node(unsigned long cpu) {}
#endif /* CONFIG_NUMA */
#if defined(CONFIG_NUMA) && defined(CONFIG_PPC_SPLPAR)
-extern int find_and_online_cpu_nid(int cpu);
+extern void find_and_update_cpu_nid(int cpu);
extern int cpu_to_coregroup_id(int cpu);
#else
-static inline int find_and_online_cpu_nid(int cpu)
-{
- return 0;
-}
-
+static inline void find_and_update_cpu_nid(int cpu) {}
static inline int cpu_to_coregroup_id(int cpu)
{
#ifdef CONFIG_SMP
diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index b9b7fefbb64b..b5bc8b1a833d 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c
@@ -1423,43 +1423,28 @@ static long vphn_get_associativity(unsigned long cpu,
return rc;
}
-int find_and_online_cpu_nid(int cpu)
+void find_and_update_cpu_nid(int cpu)
{
__be32 associativity[VPHN_ASSOC_BUFSIZE] = {0};
int new_nid;
/* Use associativity from first thread for all siblings */
if (vphn_get_associativity(cpu, associativity))
- return cpu_to_node(cpu);
+ return;
+ /* Do not have previous associativity, so find it now. */
new_nid = associativity_to_nid(associativity);
+
if (new_nid < 0 || !node_possible(new_nid))
new_nid = first_online_node;
-
- if (NODE_DATA(new_nid) == NULL) {
-#ifdef CONFIG_MEMORY_HOTPLUG
- /*
- * Need to ensure that NODE_DATA is initialized for a node from
- * available memory (see memblock_alloc_try_nid). If unable to
- * init the node, then default to nearest node that has memory
- * installed. Skip onlining a node if the subsystems are not
- * yet initialized.
- */
- if (!topology_inited || try_online_node(new_nid))
- new_nid = first_online_node;
-#else
- /*
- * Default to using the nearest node that has memory installed.
- * Otherwise, it would be necessary to patch the kernel MM code
- * to deal with more memoryless-node error conditions.
+ else
+ /* Associate node <-> cpu, so cpu_up() calls
+ * try_online_node() on the right node.
*/
- new_nid = first_online_node;
-#endif
- }
+ set_cpu_numa_node(cpu, new_nid);
pr_debug("%s:%d cpu %d nid %d\n", __FUNCTION__, __LINE__,
cpu, new_nid);
- return new_nid;
}
int cpu_to_coregroup_id(int cpu)
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index b81fc846d99c..0f8cd8b06432 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -398,7 +398,7 @@ static int dlpar_online_cpu(struct device_node *dn)
if (get_hard_smp_processor_id(cpu) != thread)
continue;
cpu_maps_update_done();
- find_and_online_cpu_nid(cpu);
+ find_and_update_cpu_nid(cpu);
rc = device_online(get_cpu_device(cpu));
if (rc) {
dlpar_offline_cpu(dn);
--
2.16.4
--
Oscar Salvador
SUSE Labs
