mm/vmscan.c: prevent useless kswapd loops

BugLink: https://bugs.launchpad.net/bugs/1838824
commit dffcac2cb88e4ec5906235d64a83d802580b119e upstream.

In production we have noticed hard lockups on large machines running
large jobs due to kswaps hoarding lru lock within isolate_lru_pages when
sc->reclaim_idx is 0 which is a small zone.  The lru was couple hundred
GiBs and the condition (page_zonenum(page) > sc->reclaim_idx) in
isolate_lru_pages() was basically skipping GiBs of pages while holding
the LRU spinlock with interrupt disabled.

On further inspection, it seems like there are two issues:

(1) If kswapd on the return from balance_pgdat() could not sleep (i.e.
    node is still unbalanced), the classzone_idx is unintentionally set
    to 0 and the whole reclaim cycle of kswapd will try to reclaim only
    the lowest and smallest zone while traversing the whole memory.

(2) Fundamentally isolate_lru_pages() is really bad when the
    allocation has woken kswapd for a smaller zone on a very large machine
    running very large jobs.  It can hoard the LRU spinlock while skipping
    over 100s of GiBs of pages.

This patch only fixes (1).  (2) needs a more fundamental solution.  To
fix (1), in the kswapd context, if pgdat->kswapd_classzone_idx is
invalid use the classzone_idx of the previous kswapd loop otherwise use
the one the waker has requested.

Link: http://lkml.kernel.org/r/20190701201847.251028-1-shakeelb@google.com
Fixes: e716f2eb24de ("mm, vmscan: prevent kswapd sleeping prematurely due to mismatched classzone_idx")
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Yang Shi <yang.shi@linux.alibaba.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 851691bef6734e23684d3541ca8b3acc75000913..9ad608ded8b51378b1db2eb367827b25b20f2d37 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3458,19 +3458,18 @@ out:
 }
 
 /*
- * pgdat->kswapd_classzone_idx is the highest zone index that a recent
- * allocation request woke kswapd for. When kswapd has not woken recently,
- * the value is MAX_NR_ZONES which is not a valid index. This compares a
- * given classzone and returns it or the highest classzone index kswapd
- * was recently woke for.
+ * The pgdat->kswapd_classzone_idx is used to pass the highest zone index to be
+ * reclaimed by kswapd from the waker. If the value is MAX_NR_ZONES which is not
+ * a valid index then either kswapd runs for first time or kswapd couldn't sleep
+ * after previous reclaim attempt (node is still unbalanced). In that case
+ * return the zone index of the previous kswapd reclaim cycle.
  */
 static enum zone_type kswapd_classzone_idx(pg_data_t *pgdat,
-                                          enum zone_type classzone_idx)
+                                          enum zone_type prev_classzone_idx)
 {
        if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
-               return classzone_idx;
-
-       return max(pgdat->kswapd_classzone_idx, classzone_idx);
+               return prev_classzone_idx;
+       return pgdat->kswapd_classzone_idx;
 }
 
 static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
@@ -3611,7 +3610,7 @@ kswapd_try_sleep:
 
                /* Read the new order and classzone_idx */
                alloc_order = reclaim_order = pgdat->kswapd_order;
-               classzone_idx = kswapd_classzone_idx(pgdat, 0);
+               classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
                pgdat->kswapd_order = 0;
                pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
 
@@ -3662,8 +3661,12 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
        if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
                return;
        pgdat = zone->zone_pgdat;
-       pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat,
-                                                          classzone_idx);
+
+       if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
+               pgdat->kswapd_classzone_idx = classzone_idx;
+       else
+               pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
+                                                 classzone_idx);
        pgdat->kswapd_order = max(pgdat->kswapd_order, order);
        if (!waitqueue_active(&pgdat->kswapd_wait))
                return;