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Commit 6098c674 authored by Johannes Weiner's avatar Johannes Weiner Committed by Vinayak Menon
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mm: vmscan: kick flushers when we encounter dirty pages on the LRU 

Memory pressure can put dirty pages at the end of the LRU without
anybody running into dirty limits.  Don't start writing individual pages
from kswapd while the flushers might be asleep.

Unlike the old direct reclaim flusher wakeup (removed in the next patch)
that flushes the number of pages just scanned, this patch wakes the
flushers for all outstanding dirty pages.  That seemed to perform better
in a synthetic test that pushes dirty pages to the end of the LRU and
into reclaim, because we know LRU aging outstrips writeback already, and
this way we give younger dirty pages a headstart rather than wait until
reclaim runs into them as well.  It also means less plugging and risk of
exhausting the struct request pool from reclaim.

There is a concern that this will cause temporary files that used to get
dirtied and truncated before writeback to now get written to disk under
memory pressure.  If this turns out to be a real problem, we'll have to
revisit this and tame the reclaim flusher wakeups.

Change-Id: I6660962957d0bd1a5f1b1f9a93ac104af6045505
[hannes@cmpxchg.org: mention dirty expiration as a condition]
  Link: http://lkml.kernel.org/r/20170126174739.GA30636@cmpxchg.org
Link: http://lkml.kernel.org/r/20170123181641.23938-3-hannes@cmpxchg.org


Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Acked-by: default avatarMinchan Kim <minchan@kernel.org>
Acked-by: default avatarMichal Hocko <mhocko@suse.com>
Acked-by: default avatarMel Gorman <mgorman@suse.de>
Acked-by: default avatarHillf Danton <hillf.zj@alibaba-inc.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Git-Commit: 726d061fbd3658e4bfeffa1b8e82da97de2ca4dd
Git-Repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


[vinmenon@codeaurora.org trivial merge conflict fixes]
Signed-off-by: default avatarVinayak Menon <vinmenon@codeaurora.org>
parent 14d6ecf6

include/linux/writeback.h

+1 −1
Original line number Diff line number Diff line
@@ -43,7 +43,7 @@ enum writeback_sync_modes { 
 */

enum wb_reason {

	WB_REASON_BACKGROUND,

	WB_REASON_TRY_TO_FREE_PAGES,

	WB_REASON_VMSCAN,

	WB_REASON_SYNC,

	WB_REASON_PERIODIC,

	WB_REASON_LAPTOP_TIMER,

include/trace/events/writeback.h

+1 −1
Original line number Diff line number Diff line
@@ -31,7 +31,7 @@ 


#define WB_WORK_REASON							\

	EM( WB_REASON_BACKGROUND,		"background")		\

	EM( WB_REASON_TRY_TO_FREE_PAGES,	"try_to_free_pages")	\

	EM( WB_REASON_VMSCAN,			"vmscan")		\

	EM( WB_REASON_SYNC,			"sync")			\

	EM( WB_REASON_PERIODIC,			"periodic")		\

	EM( WB_REASON_LAPTOP_TIMER,		"laptop_timer")		\

mm/vmscan.c

+13 −5
Original line number Diff line number Diff line
@@ -1899,12 +1899,20 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec, 


		/*

		 * If dirty pages are scanned that are not queued for IO, it

		 * implies that flushers are not keeping up. In this case, flag

		 * the pgdat PGDAT_DIRTY and kswapd will start writing pages from

		 * reclaim context.

		 */

		if (nr_unqueued_dirty == nr_taken)

		 * implies that flushers are not doing their job. This can

		 * happen when memory pressure pushes dirty pages to the end of

		 * the LRU before the dirty limits are breached and the dirty

		 * data has expired. It can also happen when the proportion of

		 * dirty pages grows not through writes but through memory

		 * pressure reclaiming all the clean cache. And in some cases,

		 * the flushers simply cannot keep up with the allocation

		 * rate. Nudge the flusher threads in case they are asleep, but

		 * also allow kswapd to start writing pages during reclaim.

		 */

		if (nr_unqueued_dirty == nr_taken) {

			wakeup_flusher_threads(0, WB_REASON_VMSCAN);

			set_bit(PGDAT_DIRTY, &pgdat->flags);

		}



		/*

		 * If kswapd scans pages marked marked for immediate
@@ -2851,7 +2859,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist, 
		writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;

		if (total_scanned > writeback_threshold) {

			wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,

						WB_REASON_TRY_TO_FREE_PAGES);

						WB_REASON_VMSCAN);

			sc->may_writepage = 1;

		}

	} while (--sc->priority >= 0);