[08/17] mm, vmstat: reduce zone->lock holding time by /proc/pagetypeinfo

Commit Message

Andrew Morton Nov. 6, 2019, 5:16 a.m. UTC

From: Michal Hocko <[e-mail]>
Subject: mm, vmstat: reduce zone->lock holding time by /proc/pagetypeinfo

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 and low number of
pages is much more interesting 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.

While we are at it, also drop the zone lock after each free_list iteration
which will help with the IRQ and page allocator responsiveness even
further as the IRQ lock held time is always bound to those 100k pages.

[[e-mail]: tweak comment text, per David Hildenbrand]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Michal Hocko <[e-mail]>
Suggested-by: Andrew Morton <[e-mail]>
Reviewed-by: Waiman Long <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Acked-by: Rafael Aquini <[e-mail]>
Acked-by: David Rientjes <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Song Liu <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/vmstat.c |   23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

Patch

--- a/mm/vmstat.c~mm-vmstat-reduce-zone-lock-holding-time-by-proc-pagetypeinfo
+++ a/mm/vmstat.c
@@ -1383,12 +1383,29 @@  static void pagetypeinfo_showfree_print(
 			unsigned long freecount = 0;
 			struct free_area *area;
 			struct list_head *curr;
+			bool overflow = false;
 
 			area = &(zone->free_area[order]);
 
-			list_for_each(curr, &area->free_list[mtype])
-				freecount++;
-			seq_printf(m, "%6lu ", freecount);
+			list_for_each(curr, &area->free_list[mtype]) {
+				/*
+				 * 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 of this order should be more than
+				 * sufficient.
+				 */
+				if (++freecount >= 100000) {
+					overflow = true;
+					break;
+				}
+			}
+			seq_printf(m, "%s%6lu ", overflow ? ">" : "", freecount);
+			spin_unlock_irq(&zone->lock);
+			cond_resched();
+			spin_lock_irq(&zone->lock);
 		}
 		seq_putc(m, '\n');
 	}