[PATCH v2 1/1] mm: fix interrupt disabled long time inside deferred_init_memmap()

[Shile Zhang <shile.zhang@linux.alibaba.com>]

When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdatinit' kthread will
initialise the deferred pages with local interrupts disabled. It is
introduced by commit 3a2d7fa8a3d5 ("mm: disable interrupts while
initializing deferred pages").

The local interrupt will be disabled long time inside
deferred_init_memmap(), depends on memory size.
On machine with NCPUS <= 2, the 'pgdatinit' kthread could be pined on
boot CPU, then the tick timer will stuck long time, which caused the
system wall time inaccuracy.

For example, the dmesg shown that:

  [    0.197975] node 0 initialised, 32170688 pages in 1ms

Obviously, 1ms is unreasonable.
Now, fix it by restore in the pending interrupts inside the while loop.
The reasonable demsg shown likes:

[    1.069306] node 0 initialised, 32203456 pages in 894ms

Signed-off-by: Shile Zhang <shile.zhang@linux.alibaba.com>
---
 mm/page_alloc.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3c4eb750a199..d3f337f2e089 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1809,8 +1809,12 @@ static int __init deferred_init_memmap(void *data)
 	 * that we can avoid introducing any issues with the buddy
 	 * allocator.
 	 */
-	while (spfn < epfn)
+	while (spfn < epfn) {
 		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
+		/* let in any pending interrupts */
+		local_irq_restore(flags);
+		local_irq_save(flags);
+	}
 zone_empty:
 	pgdat_resize_unlock(pgdat, &flags);
 
-- 
2.24.0.rc2