Commit d527caf2 authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds
Browse files

mm: compaction: prevent kswapd compacting memory to reduce CPU usage 

This patch reverts 5a03b051 ("thp: use compaction in kswapd for GFP_ATOMIC
order > 0") due to reports stating that kswapd CPU usage was higher and
IRQs were being disabled more frequently.  This was reported at
http://www.spinics.net/linux/fedora/alsa-user/msg09885.html.

Without this patch applied, CPU usage by kswapd hovers around the 20% mark
according to the tester (Arthur Marsh:
http://www.spinics.net/linux/fedora/alsa-user/msg09899.html

).  With this
patch applied, it's around 2%.

The problem is not related to THP which specifies __GFP_NO_KSWAPD but is
triggered by high-order allocations hitting the low watermark for their
order and waking kswapd on kernels with CONFIG_COMPACTION set.  The most
common trigger for this is network cards configured for jumbo frames but
it's also possible it'll be triggered by fork-heavy workloads (order-1)
and some wireless cards which depend on order-1 allocations.

The symptoms for the user will be high CPU usage by kswapd in low-memory
situations which could be confused with another writeback problem.  While
a patch like 5a03b051 may be reintroduced in the future, this patch plays
it safe for now and reverts it.

[mel@csn.ul.ie: Beefed up the changelog]
Signed-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
Reported-by: default avatarArthur Marsh <arthur.marsh@internode.on.net>
Tested-by: default avatarArthur Marsh <arthur.marsh@internode.on.net>
Cc: <stable@kernel.org>		[2.6.38.1]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 89699605

include/linux/compaction.h

+2 −7
Original line number Diff line number Diff line
@@ -11,9 +11,6 @@ 
/* The full zone was compacted */

#define COMPACT_COMPLETE	3



#define COMPACT_MODE_DIRECT_RECLAIM	0

#define COMPACT_MODE_KSWAPD		1



#ifdef CONFIG_COMPACTION

extern int sysctl_compact_memory;

extern int sysctl_compaction_handler(struct ctl_table *table, int write,
@@ -28,8 +25,7 @@ extern unsigned long try_to_compact_pages(struct zonelist *zonelist, 
			bool sync);

extern unsigned long compaction_suitable(struct zone *zone, int order);

extern unsigned long compact_zone_order(struct zone *zone, int order,

					gfp_t gfp_mask, bool sync,

					int compact_mode);

					gfp_t gfp_mask, bool sync);



/* Do not skip compaction more than 64 times */

#define COMPACT_MAX_DEFER_SHIFT 6
@@ -74,8 +70,7 @@ static inline unsigned long compaction_suitable(struct zone *zone, int order) 
}



static inline unsigned long compact_zone_order(struct zone *zone, int order,

					       gfp_t gfp_mask, bool sync,

					       int compact_mode)

					       gfp_t gfp_mask, bool sync)

{

	return COMPACT_CONTINUE;

}

mm/compaction.c

+3 −21
Original line number Diff line number Diff line
@@ -42,8 +42,6 @@ struct compact_control { 
	unsigned int order;		/* order a direct compactor needs */

	int migratetype;		/* MOVABLE, RECLAIMABLE etc */

	struct zone *zone;



	int compact_mode;

};



static unsigned long release_freepages(struct list_head *freelist)
@@ -397,10 +395,7 @@ static int compact_finished(struct zone *zone, 
		return COMPACT_COMPLETE;



	/* Compaction run is not finished if the watermark is not met */

	if (cc->compact_mode != COMPACT_MODE_KSWAPD)

	watermark = low_wmark_pages(zone);

	else

		watermark = high_wmark_pages(zone);

	watermark += (1 << cc->order);



	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
@@ -413,15 +408,6 @@ static int compact_finished(struct zone *zone, 
	if (cc->order == -1)

		return COMPACT_CONTINUE;



	/*

	 * Generating only one page of the right order is not enough

	 * for kswapd, we must continue until we're above the high

	 * watermark as a pool for high order GFP_ATOMIC allocations

	 * too.

	 */

	if (cc->compact_mode == COMPACT_MODE_KSWAPD)

		return COMPACT_CONTINUE;



	/* Direct compactor: Is a suitable page free? */

	for (order = cc->order; order < MAX_ORDER; order++) {

		/* Job done if page is free of the right migratetype */
@@ -543,8 +529,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) 


unsigned long compact_zone_order(struct zone *zone,

				 int order, gfp_t gfp_mask,

				 bool sync,

				 int compact_mode)

				 bool sync)

{

	struct compact_control cc = {

		.nr_freepages = 0,
@@ -553,7 +538,6 @@ unsigned long compact_zone_order(struct zone *zone, 
		.migratetype = allocflags_to_migratetype(gfp_mask),

		.zone = zone,

		.sync = sync,

		.compact_mode = compact_mode,

	};

	INIT_LIST_HEAD(&cc.freepages);

	INIT_LIST_HEAD(&cc.migratepages);
@@ -599,8 +583,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist, 
								nodemask) {

		int status;



		status = compact_zone_order(zone, order, gfp_mask, sync,

					    COMPACT_MODE_DIRECT_RECLAIM);

		status = compact_zone_order(zone, order, gfp_mask, sync);

		rc = max(status, rc);



		/* If a normal allocation would succeed, stop compacting */
@@ -631,7 +614,6 @@ static int compact_node(int nid) 
			.nr_freepages = 0,

			.nr_migratepages = 0,

			.order = -1,

			.compact_mode = COMPACT_MODE_DIRECT_RECLAIM,

		};



		zone = &pgdat->node_zones[zoneid];

mm/vmscan.c

+1 −17
Original line number Diff line number Diff line
@@ -2397,7 +2397,6 @@ loop_again: 
		 * cause too much scanning of the lower zones.

		 */

		for (i = 0; i <= end_zone; i++) {

			int compaction;

			struct zone *zone = pgdat->node_zones + i;

			int nr_slab;
@@ -2428,24 +2427,9 @@ loop_again: 
			sc.nr_reclaimed += reclaim_state->reclaimed_slab;

			total_scanned += sc.nr_scanned;



			compaction = 0;

			if (order &&

			    zone_watermark_ok(zone, 0,

					       high_wmark_pages(zone),

					      end_zone, 0) &&

			    !zone_watermark_ok(zone, order,

					       high_wmark_pages(zone),

					       end_zone, 0)) {

				compact_zone_order(zone,

						   order,

						   sc.gfp_mask, false,

						   COMPACT_MODE_KSWAPD);

				compaction = 1;

			}



			if (zone->all_unreclaimable)

				continue;

			if (!compaction && nr_slab == 0 &&

			if (nr_slab == 0 &&

			    !zone_reclaimable(zone))

				zone->all_unreclaimable = 1;

			/*

CRA Git | Maintained and supported by SUSTech CRA and CCSE