Commit 602605a4 authored Mar 22, 2011 by Mel Gorman Committed by Linus Torvalds Mar 22, 2011

mm: compaction: minimise the time IRQs are disabled while isolating free pages

compaction_alloc() isolates free pages to be used as migration targets.
While its scanning, IRQs are disabled on the mistaken assumption the
scanning should be short. Analysis showed that IRQs were in fact being
disabled for substantial time. A simple test was run using large
anonymous mappings with transparent hugepage support enabled to trigger
frequent compactions. A monitor sampled what the worst IRQ-off latencies
were and a post-processing tool found the following;

Total sampled time IRQs off (not real total time): 22355
Event compaction_alloc..compaction_alloc 8409 us count 1
Event compaction_alloc..compaction_alloc 7341 us count 1
Event compaction_alloc..compaction_alloc 2463 us count 1
Event compaction_alloc..compaction_alloc 2054 us count 1
Event shrink_inactive_list..shrink_zone 1864 us count 1
Event shrink_inactive_list..shrink_zone 88 us count 1
Event save_args..call_softirq 36 us count 1
Event save_args..call_softirq 35 us count 2
Event __make_request..__blk_run_queue 24 us count 1
Event __alloc_pages_nodemask..__alloc_pages_nodemask 6 us count 1

i.e. compaction is disabled IRQs for a prolonged period of time - 8ms in
one instance. The full report generated by the tool can be found at

http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-vanilla-micro.report

This patch reduces the time IRQs are disabled by simply disabling IRQs at
the last possible minute. An updated IRQs-off summary report then looks
like;

Total sampled time IRQs off (not real total time): 5493
Event shrink_inactive_list..shrink_zone 1596 us count 1
Event shrink_inactive_list..shrink_zone 1530 us count 1
Event shrink_inactive_list..shrink_zone 956 us count 1
Event shrink_inactive_list..shrink_zone 541 us count 1
Event shrink_inactive_list..shrink_zone 531 us count 1
Event split_huge_page..add_to_swap 232 us count 1
Event save_args..call_softirq 36 us count 1
Event save_args..call_softirq 35 us count 2
Event __wake_up..__wake_up 1 us count 1

A full report is again available at

http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-minimiseirq-free-v1r4-micro.report

As should be obvious, IRQ disabled latencies due to compaction are
almost elimimnated for this particular test.

[aarcange@redhat.com: Fix initialisation of isolated]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Arthur Marsh <arthur.marsh@internode.on.net>
Cc: Clemens Ladisch <cladisch@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 5b280c0c

mm/compaction.c

+13 −5

Original line number	Diff line number	Diff line
		@@ -153,7 +153,6 @@ static void isolate_freepages(struct zone *zone,
		* pages on cc->migratepages. We stop searching if the migrate
		* and free page scanners meet or enough free pages are isolated.
		*/
		spin_lock_irqsave(&zone->lock, flags);
		for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages;
		pfn -= pageblock_nr_pages) {
		unsigned long isolated;
		@@ -176,9 +175,19 @@ static void isolate_freepages(struct zone *zone,
		if (!suitable_migration_target(page))
		continue;

		/* Found a block suitable for isolating free pages from */
		/*
		* Found a block suitable for isolating free pages from. Now
		* we disabled interrupts, double check things are ok and
		* isolate the pages. This is to minimise the time IRQs
		* are disabled
		*/
		isolated = 0;
		spin_lock_irqsave(&zone->lock, flags);
		if (suitable_migration_target(page)) {
		isolated = isolate_freepages_block(zone, pfn, freelist);
		nr_freepages += isolated;
		}
		spin_unlock_irqrestore(&zone->lock, flags);

		/*
		* Record the highest PFN we isolated pages from. When next
		@@ -188,7 +197,6 @@ static void isolate_freepages(struct zone *zone,
		if (isolated)
		high_pfn = max(high_pfn, pfn);
		}
		spin_unlock_irqrestore(&zone->lock, flags);

		/* split_free_page does not map the pages */
		list_for_each_entry(page, freelist, lru) {

Admin message