mm/vmscan.c: prevent useless kswapd loops (dffcac2c) · Commits · 戴 / test

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: ("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>

mm/vmscan.c

+15 −12

Original line number	Diff line number	Diff line
		@@ -3644,19 +3644,18 @@ 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,
		@@ -3797,7 +3796,7 @@ 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;

		@@ -3851,7 +3850,11 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
		if (!cpuset_zone_allowed(zone, gfp_flags))
		return;
		pgdat = zone->zone_pgdat;
		pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat,

		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))

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