[RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by /proc/pagetypeinfo

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

pagetypeinfo_showfree_print is called by zone->lock held in irq mode.
This is not really nice because it blocks both any interrupts on that
cpu and the page allocator. On large machines this might even trigger
the hard lockup detector.

Considering the pagetypeinfo is a debugging tool we do not really need
exact numbers here. The primary reason to look at the outuput is to see
how pageblocks are spread among different migratetypes therefore putting
a bound on the number of pages on the free_list sounds like a reasonable
tradeoff.

The new output will simply tell
[...]
Node    6, zone   Normal, type      Movable >100000 >100000 >100000 >100000  41019  31560  23996  10054   3229    983    648

instead of
Node    6, zone   Normal, type      Movable 399568 294127 221558 102119  41019  31560  23996  10054   3229    983    648

The limit has been chosen arbitrary and it is a subject of a future
change should there be a need for that.

Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/vmstat.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 4e885ecd44d1..762034fc3b83 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1386,8 +1386,25 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
 
 			area = &(zone->free_area[order]);
 
-			list_for_each(curr, &area->free_list[mtype])
+			list_for_each(curr, &area->free_list[mtype]) {
 				freecount++;
+				/*
+				 * Cap the free_list iteration because it might
+				 * be really large and we are under a spinlock
+				 * so a long time spent here could trigger a
+				 * hard lockup detector. Anyway this is a
+				 * debugging tool so knowing there is a handful
+				 * of pages in this order should be more than
+				 * sufficient
+				 */
+				if (freecount > 100000) {
+					seq_printf(m, ">%6lu ", freecount);
+					spin_unlock_irq(&zone->lock);
+					cond_resched();
+					spin_lock_irq(&zone->lock);
+					continue;
+				}
+			}
 			seq_printf(m, "%6lu ", freecount);
 		}
 		seq_putc(m, '\n');
-- 
2.20.1