There is an issue currently where NUMA information is used on powerpc
(and possibly ia64) before it has been read from the device-tree, which
leads to large slab consumption with CONFIG_SLUB and memoryless nodes.

NUMA powerpc non-boot CPU's cpu_to_node/cpu_to_mem is only accurate
after start_secondary(), similar to ia64, which is invoked via
smp_init().

Commit 6ee0578b4daae ("workqueue: mark init_workqueues() as
early_initcall()") made init_workqueues() be invoked via
do_pre_smp_initcalls(), which is obviously before the secondary
processors are online.

Additionally, the following commits changed init_workqueues() to use
cpu_to_node to determine the node to use for kthread_create_on_node:

bce903809ab3f ("workqueue: add wq_numa_tbl_len and
wq_numa_possible_cpumask[]")
f3f90ad469342 ("workqueue: determine NUMA node of workers accourding to
the allowed cpumask")

Therefore, when init_workqueues() runs, it sees all CPUs as being on
Node 0. On LPARs or KVM guests where Node 0 is memoryless, this leads to
a high number of slab deactivations
(http://www.spinics.net/lists/linux-mm/msg67489.html).

While testing memoryless nodes on PowerKVM guests with a fix to the
workqueue logic to use cpu_to_mem() instead of cpu_to_node(), with a
guest topology:

    available: 2 nodes (0-1)
    node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2
    node 0 size: 0 MB
    node 0 free: 0 MB
    node 1 cpus: 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70
    node 1 size: 16336 MB
    node 1 free: 15329 MB
    node distances:
    node   0   1
      0:  10  40
      1:  40  10

the slab consumption decreases from:

    Slab:             932416 kB
    SUnreclaim:       902336 kB

to

    Slab:             395264 kB
    SUnreclaim:       359424 kB

And we see a corresponding increase in the slab efficiency from:

    slab                                   mem     objs    slabs
                                          used   active   active
    ------------------------------------------------------------
    kmalloc-16384                       337 MB   11.28%  100.00%
    task_struct                         288 MB    9.93%  100.00%

to:

    slab                                   mem     objs    slabs
                                          used   active   active
    ------------------------------------------------------------
    kmalloc-16384                        37 MB  100.00%  100.00%
    task_struct                          31 MB  100.00%  100.00%

Powerpc didn't support memoryless nodes until recently (64bb80d87f01
"powerpc/numa: Enable CONFIG_HAVE_MEMORYLESS_NODES" and 8c272261194d
"powerpc/numa: Enable USE_PERCPU_NUMA_NODE_ID"). Those commits also
helped improve memory consumption with these kind of environments.

Signed-off-by: Nishanth Aravamudan <redacted>

---
Ben & others, one area I'm still unsure of is if calling the NUMA
callback for all CPUs is desired. I don't know how else to get the NUMA
topology into the array easily, but I didn't test in an environment with
hotpluggable CPUs, so I'm not sure if it will lead to errors there (are
there device-tree entries for the topology of CPUs that will be plugged
in? I assume not, actually, so maybe we should keep the logic in
start_secondary so that those CPUs that are hotplugged later get the
right topology data?

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 1007fb802e6b..1fc8984f272e 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c

@@ -376,6 +376,12 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 					GFP_KERNEL, cpu_to_node(cpu));
 		zalloc_cpumask_var_node(&per_cpu(cpu_core_map, cpu),
 					GFP_KERNEL, cpu_to_node(cpu));
+		/*
+		 * numa_node_id() works after this.
+		 */
+		set_cpu_numa_node(cpu, numa_cpu_lookup_table[cpu]);
+		set_cpu_numa_mem(cpu,
+				 local_memory_node(numa_cpu_lookup_table[cpu]));
 	}
 
 	cpumask_set_cpu(boot_cpuid, cpu_sibling_mask(boot_cpuid));

@@ -723,12 +729,6 @@ void start_secondary(void *unused)
 	}
 	traverse_core_siblings(cpu, true);
 
-	/*
-	 * numa_node_id() works after this.
-	 */
-	set_numa_node(numa_cpu_lookup_table[cpu]);
-	set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]));
-
 	smp_wmb();
 	notify_cpu_starting(cpu);
 	set_cpu_online(cpu, true);

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index d3e9a78eaed3..32341e16b8ce 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c

@@ -1049,7 +1049,7 @@ static void __init mark_reserved_regions_for_nid(int nid)
 
 void __init do_init_bootmem(void)
 {
-	int nid;
+	int nid, cpu;
 
 	min_low_pfn = 0;
 	max_low_pfn = memblock_end_of_DRAM() >> PAGE_SHIFT;

@@ -1122,8 +1122,15 @@ void __init do_init_bootmem(void)
 
 	reset_numa_cpu_lookup_table();
 	register_cpu_notifier(&ppc64_numa_nb);
-	cpu_numa_callback(&ppc64_numa_nb, CPU_UP_PREPARE,
-			  (void *)(unsigned long)boot_cpuid);
+	/*
+	 * We need the numa_cpu_lookup_table to be accurate for all
+	 * CPUs, even before we online them, so that we can use
+	 * cpu_to_{node,mem} early in boot, cf. smp_prepare_cpus().
+	 */
+	for_each_possible_cpu(cpu) {
+		cpu_numa_callback(&ppc64_numa_nb, CPU_UP_PREPARE,
+				  (void *)(unsigned long)boot_cpuid);
+	}
 }
 
 void __init paging_init(void)

--

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