Commit 5515061d authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
Browse files

mm: throttle direct reclaimers if PF_MEMALLOC reserves are low and swap is... 


mm: throttle direct reclaimers if PF_MEMALLOC reserves are low and swap is backed by network storage

If swap is backed by network storage such as NBD, there is a risk that a
large number of reclaimers can hang the system by consuming all
PF_MEMALLOC reserves.  To avoid these hangs, the administrator must tune
min_free_kbytes in advance which is a bit fragile.

This patch throttles direct reclaimers if half the PF_MEMALLOC reserves
are in use.  If the system is routinely getting throttled the system
administrator can increase min_free_kbytes so degradation is smoother but
the system will keep running.

Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
Cc: David Miller <davem@davemloft.net>
Cc: Neil Brown <neilb@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Cc: Eric B Munson <emunson@mgebm.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7f338fe4

include/linux/mmzone.h

+1 −0
Original line number Diff line number Diff line
@@ -705,6 +705,7 @@ typedef struct pglist_data { 
					     range, including holes */

	int node_id;

	wait_queue_head_t kswapd_wait;

	wait_queue_head_t pfmemalloc_wait;

	struct task_struct *kswapd;	/* Protected by lock_memory_hotplug() */

	int kswapd_max_order;

	enum zone_type classzone_idx;

mm/page_alloc.c

+1 −0
Original line number Diff line number Diff line
@@ -4389,6 +4389,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat, 
	pgdat_resize_init(pgdat);

	pgdat->nr_zones = 0;

	init_waitqueue_head(&pgdat->kswapd_wait);

	init_waitqueue_head(&pgdat->pfmemalloc_wait);

	pgdat->kswapd_max_order = 0;

	pgdat_page_cgroup_init(pgdat);

mm/vmscan.c

+120 −8
Original line number Diff line number Diff line
@@ -2112,6 +2112,80 @@ out: 
	return 0;

}



static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)

{

	struct zone *zone;

	unsigned long pfmemalloc_reserve = 0;

	unsigned long free_pages = 0;

	int i;

	bool wmark_ok;



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

		zone = &pgdat->node_zones[i];

		pfmemalloc_reserve += min_wmark_pages(zone);

		free_pages += zone_page_state(zone, NR_FREE_PAGES);

	}



	wmark_ok = free_pages > pfmemalloc_reserve / 2;



	/* kswapd must be awake if processes are being throttled */

	if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {

		pgdat->classzone_idx = min(pgdat->classzone_idx,

						(enum zone_type)ZONE_NORMAL);

		wake_up_interruptible(&pgdat->kswapd_wait);

	}



	return wmark_ok;

}



/*

 * Throttle direct reclaimers if backing storage is backed by the network

 * and the PFMEMALLOC reserve for the preferred node is getting dangerously

 * depleted. kswapd will continue to make progress and wake the processes

 * when the low watermark is reached

 */

static void throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,

					nodemask_t *nodemask)

{

	struct zone *zone;

	int high_zoneidx = gfp_zone(gfp_mask);

	pg_data_t *pgdat;



	/*

	 * Kernel threads should not be throttled as they may be indirectly

	 * responsible for cleaning pages necessary for reclaim to make forward

	 * progress. kjournald for example may enter direct reclaim while

	 * committing a transaction where throttling it could forcing other

	 * processes to block on log_wait_commit().

	 */

	if (current->flags & PF_KTHREAD)

		return;



	/* Check if the pfmemalloc reserves are ok */

	first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);

	pgdat = zone->zone_pgdat;

	if (pfmemalloc_watermark_ok(pgdat))

		return;



	/*

	 * If the caller cannot enter the filesystem, it's possible that it

	 * is due to the caller holding an FS lock or performing a journal

	 * transaction in the case of a filesystem like ext[3|4]. In this case,

	 * it is not safe to block on pfmemalloc_wait as kswapd could be

	 * blocked waiting on the same lock. Instead, throttle for up to a

	 * second before continuing.

	 */

	if (!(gfp_mask & __GFP_FS)) {

		wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,

			pfmemalloc_watermark_ok(pgdat), HZ);

		return;

	}



	/* Throttle until kswapd wakes the process */

	wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,

		pfmemalloc_watermark_ok(pgdat));

}



unsigned long try_to_free_pages(struct zonelist *zonelist, int order,

				gfp_t gfp_mask, nodemask_t *nodemask)

{
@@ -2131,6 +2205,15 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order, 
		.gfp_mask = sc.gfp_mask,

	};



	throttle_direct_reclaim(gfp_mask, zonelist, nodemask);



	/*

	 * Do not enter reclaim if fatal signal is pending. 1 is returned so

	 * that the page allocator does not consider triggering OOM

	 */

	if (fatal_signal_pending(current))

		return 1;



	trace_mm_vmscan_direct_reclaim_begin(order,

				sc.may_writepage,

				gfp_mask);
@@ -2275,8 +2358,13 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages, 
	return balanced_pages >= (present_pages >> 2);

}



/* is kswapd sleeping prematurely? */

static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,

/*

 * Prepare kswapd for sleeping. This verifies that there are no processes

 * waiting in throttle_direct_reclaim() and that watermarks have been met.

 *

 * Returns true if kswapd is ready to sleep

 */

static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,

					int classzone_idx)

{

	int i;
@@ -2285,7 +2373,21 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining, 


	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */

	if (remaining)

		return true;

		return false;



	/*

	 * There is a potential race between when kswapd checks its watermarks

	 * and a process gets throttled. There is also a potential race if

	 * processes get throttled, kswapd wakes, a large process exits therby

	 * balancing the zones that causes kswapd to miss a wakeup. If kswapd

	 * is going to sleep, no process should be sleeping on pfmemalloc_wait

	 * so wake them now if necessary. If necessary, processes will wake

	 * kswapd and get throttled again

	 */

	if (waitqueue_active(&pgdat->pfmemalloc_wait)) {

		wake_up(&pgdat->pfmemalloc_wait);

		return false;

	}



	/* Check the watermark levels */

	for (i = 0; i <= classzone_idx; i++) {
@@ -2318,9 +2420,9 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining, 
	 * must be balanced

	 */

	if (order)

		return !pgdat_balanced(pgdat, balanced, classzone_idx);

		return pgdat_balanced(pgdat, balanced, classzone_idx);

	else

		return !all_zones_ok;

		return all_zones_ok;

}



/*
@@ -2546,6 +2648,16 @@ loop_again: 
			}



		}



		/*

		 * If the low watermark is met there is no need for processes

		 * to be throttled on pfmemalloc_wait as they should not be

		 * able to safely make forward progress. Wake them

		 */

		if (waitqueue_active(&pgdat->pfmemalloc_wait) &&

				pfmemalloc_watermark_ok(pgdat))

			wake_up(&pgdat->pfmemalloc_wait);



		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))

			break;		/* kswapd: all done */

		/*
@@ -2647,7 +2759,7 @@ out: 
	}



	/*

	 * Return the order we were reclaiming at so sleeping_prematurely()

	 * Return the order we were reclaiming at so prepare_kswapd_sleep()

	 * makes a decision on the order we were last reclaiming at. However,

	 * if another caller entered the allocator slow path while kswapd

	 * was awake, order will remain at the higher level
@@ -2667,7 +2779,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx) 
	prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);



	/* Try to sleep for a short interval */

	if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {

	if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {

		remaining = schedule_timeout(HZ/10);

		finish_wait(&pgdat->kswapd_wait, &wait);

		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
@@ -2677,7 +2789,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx) 
	 * After a short sleep, check if it was a premature sleep. If not, then

	 * go fully to sleep until explicitly woken up.

	 */

	if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {

	if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {

		trace_mm_vmscan_kswapd_sleep(pgdat->node_id);



		/*

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