[PATCH 1/3] mm, memory_hotplug: add scheduling point to __add_pages

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

__add_pages gets a pfn range to add and there is no upper bound for a
single call. This is usually a memory block aligned size for the regular
memory hotplug - smaller sizes are usual for memory balloning drivers,
or the whole NUMA node for physical memory online. There is no explicit
scheduling point in that code path though.

This can lead to long latencies while __add_pages is executed and we
have even seen a soft lockup report during nvdimm initialization with
!PREEMPT kernel

[   33.588806] NMI watchdog: BUG: soft lockup - CPU#11 stuck for 23s! [kworker/u641:3:832]
[...]
[   33.588875] Workqueue: events_unbound async_run_entry_fn
[   33.588876] task: ffff881809270f40 ti: ffff881809274000 task.ti: ffff881809274000
[   33.588878] RIP: 0010:[<ffffffff81608c01>]  [<ffffffff81608c01>] _raw_spin_unlock_irqrestore+0x11/0x20
[   33.588883] RSP: 0018:ffff881809277b10  EFLAGS: 00000286
[...]
[   33.588900] Call Trace:
[   33.588906]  [<ffffffff81603a45>] sparse_add_one_section+0x13d/0x18e
[   33.588909]  [<ffffffff815fde8a>] __add_pages+0x10a/0x1d0
[   33.588916]  [<ffffffff810634ca>] arch_add_memory+0x4a/0xc0
[   33.588920]  [<ffffffff8118b22d>] devm_memremap_pages+0x29d/0x430
[   33.588931]  [<ffffffffa042e50d>] pmem_attach_disk+0x2fd/0x3f0 [nd_pmem]
[   33.589001]  [<ffffffffa14ad984>] nvdimm_bus_probe+0x64/0x110 [libnvdimm]
[   33.589008]  [<ffffffff8146a337>] driver_probe_device+0x1f7/0x420
[   33.589012]  [<ffffffff814682f2>] bus_for_each_drv+0x52/0x80
[   33.589014]  [<ffffffff8146a020>] __device_attach+0xb0/0x130
[   33.589017]  [<ffffffff81469447>] bus_probe_device+0x87/0xa0
[   33.589020]  [<ffffffff8146741c>] device_add+0x3fc/0x5f0
[   33.589029]  [<ffffffffa14acffe>] nd_async_device_register+0xe/0x40 [libnvdimm]
[   33.589047]  [<ffffffff8109e1c3>] async_run_entry_fn+0x43/0x150
[   33.589073]  [<ffffffff8109594e>] process_one_work+0x14e/0x410
[   33.589086]  [<ffffffff810961a6>] worker_thread+0x116/0x490
[   33.589089]  [<ffffffff8109b677>] kthread+0xc7/0xe0
[   33.589119]  [<ffffffff816094bf>] ret_from_fork+0x3f/0x70
[   33.590756] DWARF2 unwinder stuck at ret_from_fork+0x3f/0x70

Fix this by adding cond_resched once per each memory section in the
given pfn range. Each section is constant amount of work which itself is
not too expensive but many of them will just add up.

Reported-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/memory_hotplug.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 459bbc182d10..73b56fa49b6f 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -328,6 +328,7 @@ int __ref __add_pages(int nid, unsigned long phys_start_pfn,
 		if (err && (err != -EEXIST))
 			break;
 		err = 0;
+		cond_resched();
 	}
 	vmemmap_populate_print_last();
 out:
-- 
2.14.1