linux.git/mm/compaction.c, branch v3.7-rc7

mm: compaction: correct the nr_strict va isolated check for CMA

2012-10-19T21:07:47+00:00

Thierry reported that the "iron out" patch for isolate_freepages_block()
had problems due to the strict check being too strict with "mm:
compaction: Iron out isolate_freepages_block() and
isolate_freepages_range() -fix1".  It's possible that more pages than
necessary are isolated but the check still fails and I missed that this
fix was not picked up before RC1.  This same problem has been identified
in 3.7-RC1 by Tony Prisk and should be addressed by the following patch.

Signed-off-by: Mel Gorman 
Tested-by: Tony Prisk 
Reported-by: Thierry Reding 
Acked-by: Rik van Riel 
Acked-by: Minchan Kim 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Cc: Arnd Bergmann 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

CMA: migrate mlocked pages

2012-10-09T07:23:00+00:00

Presently CMA cannot migrate mlocked pages so it ends up failing to allocate
contiguous memory space.

This patch makes mlocked pages be migrated out.  Of course, it can affect
realtime processes but in CMA usecase, contiguous memory allocation failing
is far worse than access latency to an mlocked page being variable while
CMA is running.  If someone wants to make the system realtime, he shouldn't
enable CMA because stalls can still happen at random times.

[akpm@linux-foundation.org: tweak comment text, per Mel]
Signed-off-by: Minchan Kim 
Acked-by: Mel Gorman 
Cc: Michal Nazarewicz 
Cc: Bartlomiej Zolnierkiewicz 
Cc: Marek Szyprowski 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: clear PG_migrate_skip based on compaction and reclaim activity

2012-10-09T07:22:51+00:00

Compaction caches if a pageblock was scanned and no pages were isolated so
that the pageblocks can be skipped in the future to reduce scanning.  This
information is not cleared by the page allocator based on activity due to
the impact it would have to the page allocator fast paths.  Hence there is
a requirement that something clear the cache or pageblocks will be skipped
forever.  Currently the cache is cleared if there were a number of recent
allocation failures and it has not been cleared within the last 5 seconds.
Time-based decisions like this are terrible as they have no relationship
to VM activity and is basically a big hammer.

Unfortunately, accurate heuristics would add cost to some hot paths so
this patch implements a rough heuristic.  There are two cases where the
cache is cleared.

1. If a !kswapd process completes a compaction cycle (migrate and free
   scanner meet), the zone is marked compact_blockskip_flush. When kswapd
   goes to sleep, it will clear the cache. This is expected to be the
   common case where the cache is cleared. It does not really matter if
   kswapd happens to be asleep or going to sleep when the flag is set as
   it will be woken on the next allocation request.

2. If there have been multiple failures recently and compaction just
   finished being deferred then a process will clear the cache and start a
   full scan.  This situation happens if there are multiple high-order
   allocation requests under heavy memory pressure.

The clearing of the PG_migrate_skip bits and other scans is inherently
racy but the race is harmless.  For allocations that can fail such as THP,
they will simply fail.  For requests that cannot fail, they will retry the
allocation.  Tests indicated that scanning rates were roughly similar to
when the time-based heuristic was used and the allocation success rates
were similar.

Signed-off-by: Mel Gorman 
Cc: Rik van Riel 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Cc: Rafael Aquini 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: Restart compaction from near where it left off

2012-10-09T07:22:50+00:00

This is almost entirely based on Rik's previous patches and discussions
with him about how this might be implemented.

Order > 0 compaction stops when enough free pages of the correct page
order have been coalesced.  When doing subsequent higher order
allocations, it is possible for compaction to be invoked many times.

However, the compaction code always starts out looking for things to
compact at the start of the zone, and for free pages to compact things to
at the end of the zone.

This can cause quadratic behaviour, with isolate_freepages starting at the
end of the zone each time, even though previous invocations of the
compaction code already filled up all free memory on that end of the zone.
 This can cause isolate_freepages to take enormous amounts of CPU with
certain workloads on larger memory systems.

This patch caches where the migration and free scanner should start from
on subsequent compaction invocations using the pageblock-skip information.
 When compaction starts it begins from the cached restart points and will
update the cached restart points until a page is isolated or a pageblock
is skipped that would have been scanned by synchronous compaction.

Signed-off-by: Mel Gorman 
Acked-by: Rik van Riel 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Acked-by: Rafael Aquini 
Cc: Fengguang Wu 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: cache if a pageblock was scanned and no pages were isolated

2012-10-09T07:22:50+00:00

When compaction was implemented it was known that scanning could
potentially be excessive.  The ideal was that a counter be maintained for
each pageblock but maintaining this information would incur a severe
penalty due to a shared writable cache line.  It has reached the point
where the scanning costs are a serious problem, particularly on
long-lived systems where a large process starts and allocates a large
number of THPs at the same time.

Instead of using a shared counter, this patch adds another bit to the
pageblock flags called PG_migrate_skip.  If a pageblock is scanned by
either migrate or free scanner and 0 pages were isolated, the pageblock is
marked to be skipped in the future.  When scanning, this bit is checked
before any scanning takes place and the block skipped if set.

The main difficulty with a patch like this is "when to ignore the cached
information?" If it's ignored too often, the scanning rates will still be
excessive.  If the information is too stale then allocations will fail
that might have otherwise succeeded.  In this patch

o CMA always ignores the information
o If the migrate and free scanner meet then the cached information will
  be discarded if it's at least 5 seconds since the last time the cache
  was discarded
o If there are a large number of allocation failures, discard the cache.

The time-based heuristic is very clumsy but there are few choices for a
better event.  Depending solely on multiple allocation failures still
allows excessive scanning when THP allocations are failing in quick
succession due to memory pressure.  Waiting until memory pressure is
relieved would cause compaction to continually fail instead of using
reclaim/compaction to try allocate the page.  The time-based mechanism is
clumsy but a better option is not obvious.

Signed-off-by: Mel Gorman 
Acked-by: Rik van Riel 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Acked-by: Rafael Aquini 
Cc: Fengguang Wu 
Cc: Michal Nazarewicz 
Cc: Bartlomiej Zolnierkiewicz 
Cc: Kyungmin Park 
Cc: Mark Brown 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

revert "mm: have order > 0 compaction start off where it left"

2012-10-09T07:22:50+00:00

This reverts commit 7db8889ab05b ("mm: have order > 0 compaction start
off where it left") and commit de74f1cc ("mm: have order > 0 compaction
start near a pageblock with free pages").  These patches were a good
idea and tests confirmed that they massively reduced the amount of
scanning but the implementation is complex and tricky to understand.  A
later patch will cache what pageblocks should be skipped and
reimplements the concept of compact_cached_free_pfn on top for both
migration and free scanners.

Signed-off-by: Mel Gorman 
Acked-by: Rik van Riel 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Acked-by: Rafael Aquini 
Acked-by: Minchan Kim 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: acquire the zone->lock as late as possible

2012-10-09T07:22:49+00:00

Compaction's free scanner acquires the zone->lock when checking for
PageBuddy pages and isolating them.  It does this even if there are no
PageBuddy pages in the range.

This patch defers acquiring the zone lock for as long as possible.  In the
event there are no free pages in the pageblock then the lock will not be
acquired at all which reduces contention on zone->lock.

Signed-off-by: Mel Gorman 
Acked-by: Rik van Riel 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Acked-by: Rafael Aquini 
Acked-by: Minchan Kim 
Tested-by: Peter Ujfalusi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: acquire the zone->lru_lock as late as possible

2012-10-09T07:22:49+00:00

Richard Davies and Shaohua Li have both reported lock contention problems
in compaction on the zone and LRU locks as well as significant amounts of
time being spent in compaction.  This series aims to reduce lock
contention and scanning rates to reduce that CPU usage.  Richard reported
at https://lkml.org/lkml/2012/9/21/91 that this series made a big
different to a problem he reported in August:

   http://marc.info/?l=kvm&m=134511507015614&w=2

Patch 1 defers acquiring the zone->lru_lock as long as possible.

Patch 2 defers acquiring the zone->lock as lock as possible.

Patch 3 reverts Rik's "skip-free" patches as the core concept gets
	reimplemented later and the remaining patches are easier to
	understand if this is reverted first.

Patch 4 adds a pageblock-skip bit to the pageblock flags to cache what
	pageblocks should be skipped by the migrate and free scanners.
	This drastically reduces the amount of scanning compaction has
	to do.

Patch 5 reimplements something similar to Rik's idea except it uses the
	pageblock-skip information to decide where the scanners should
	restart from and does not need to wrap around.

I tested this on 3.6-rc6 + linux-next/akpm. Kernels tested were

akpm-20120920	3.6-rc6 + linux-next/akpm as of Septeber 20th, 2012
lesslock	Patches 1-6
revert		Patches 1-7
cachefail	Patches 1-8
skipuseless	Patches 1-9

Stress high-order allocation tests looked ok.  Success rates are more or
less the same with the full series applied but there is an expectation
that there is less opportunity to race with other allocation requests if
there is less scanning.  The time to complete the tests did not vary that
much and are uninteresting as were the vmstat statistics so I will not
present them here.

Using ftrace I recorded how much scanning was done by compaction and got this

                            3.6.0-rc6     3.6.0-rc6   3.6.0-rc6  3.6.0-rc6 3.6.0-rc6
                            akpm-20120920 lockless  revert-v2r2  cachefail skipuseless

Total   free    scanned         360753976  515414028  565479007   17103281   18916589
Total   free    isolated          2852429    3597369    4048601     670493     727840
Total   free    efficiency        0.0079%    0.0070%    0.0072%    0.0392%    0.0385%
Total   migrate scanned         247728664  822729112 1004645830   17946827   14118903
Total   migrate isolated          2555324    3245937    3437501     616359     658616
Total   migrate efficiency        0.0103%    0.0039%    0.0034%    0.0343%    0.0466%

The efficiency is worthless because of the nature of the test and the
number of failures.  The really interesting point as far as this patch
series is concerned is the number of pages scanned.  Note that reverting
Rik's patches massively increases the number of pages scanned indicating
that those patches really did make a difference to CPU usage.

However, caching what pageblocks should be skipped has a much higher
impact.  With patches 1-8 applied, free page and migrate page scanning are
both reduced by 95% in comparison to the akpm kernel.  If the basic
concept of Rik's patches are implemened on top then scanning then the free
scanner barely changed but migrate scanning was further reduced.  That
said, tests on 3.6-rc5 indicated that the last patch had greater impact
than what was measured here so it is a bit variable.

One way or the other, this series has a large impact on the amount of
scanning compaction does when there is a storm of THP allocations.

This patch:

Compaction's migrate scanner acquires the zone->lru_lock when scanning a
range of pages looking for LRU pages to acquire.  It does this even if
there are no LRU pages in the range.  If multiple processes are compacting
then this can cause severe locking contention.  To make matters worse
commit b2eef8c0 ("mm: compaction: minimise the time IRQs are disabled
while isolating pages for migration") releases the lru_lock every
SWAP_CLUSTER_MAX pages that are scanned.

This patch makes two changes to how the migrate scanner acquires the LRU
lock.  First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages
if the lock is contended.  This reduces the number of times it
unnecessarily disables and re-enables IRQs.  The second is that it defers
acquiring the LRU lock for as long as possible.  If there are no LRU pages
or the only LRU pages are transhuge then the LRU lock will not be acquired
at all which reduces contention on zone->lru_lock.

[minchan@kernel.org: augment comment]
[akpm@linux-foundation.org: tweak comment text]
Signed-off-by: Mel Gorman 
Acked-by: Rik van Riel 
Cc: Richard Davies 
Cc: Shaohua Li 
Cc: Avi Kivity 
Acked-by: Rafael Aquini 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: Update try_to_compact_pages()kerneldoc comment

2012-10-09T07:22:49+00:00

Parameters were added without documentation, tut tut.

Signed-off-by: Mel Gorman 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

mm: compaction: move fatal signal check out of compact_checklock_irqsave

2012-10-09T07:22:48+00:00

Commit c67fe3752abe ("mm: compaction: Abort async compaction if locks
are contended or taking too long") addressed a lock contention problem
in compaction by introducing compact_checklock_irqsave() that effecively
aborting async compaction in the event of compaction.

To preserve existing behaviour it also moved a fatal_signal_pending()
check into compact_checklock_irqsave() but that is very misleading.  It
"hides" the check within a locking function but has nothing to do with
locking as such.  It just happens to work in a desirable fashion.

This patch moves the fatal_signal_pending() check to
isolate_migratepages_range() where it belongs.  Arguably the same check
should also happen when isolating pages for freeing but it's overkill.

Signed-off-by: Mel Gorman 
Cc: Rik van Riel 
Cc: KAMEZAWA Hiroyuki 
Cc: Shaohua Li 
Cc: Minchan Kim 
Cc: Andrea Arcangeli 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds