Dne 15.10.2012 13:09, Mel Gorman napsal(a):

On Mon, Oct 15, 2012 at 11:54:13AM +0200, Jiri Slaby wrote:

quoted

On 10/12/2012 03:57 PM, Mel Gorman wrote:

quoted

mm: vmscan: scale number of pages reclaimed by reclaim/compaction only in direct reclaim

Jiri Slaby reported the following:

	(It's an effective revert of "mm: vmscan: scale number of pages
	reclaimed by reclaim/compaction based on failures".)
	Given kswapd had hours of runtime in ps/top output yesterday in the
	morning and after the revert it's now 2 minutes in sum for the last 24h,
	I would say, it's gone.

The intention of the patch in question was to compensate for the loss of
lumpy reclaim. Part of the reason lumpy reclaim worked is because it
aggressively reclaimed pages and this patch was meant to be a
sane compromise.

When compaction fails, it gets deferred and both compaction and
reclaim/compaction is deferred avoid excessive reclaim. However, since
commit c6543459 (mm: remove __GFP_NO_KSWAPD), kswapd is woken up each time
and continues reclaiming which was not taken into account when the patch
was developed.

As it is not taking deferred compaction into account in this path it scans
aggressively before falling out and making the compaction_deferred check in
compaction_ready. This patch avoids kswapd scaling pages for reclaim and
leaves the aggressive reclaim to the process attempting the THP
allocation.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
  mm/vmscan.c |   10 ++++++++--
  1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2624edc..2b7edfa 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -1763,14 +1763,20 @@ static bool in_reclaim_compaction(struct scan_control *sc)
  #ifdef CONFIG_COMPACTION
  /*
   * If compaction is deferred for sc->order then scale the number of pages
- * reclaimed based on the number of consecutive allocation failures
+ * reclaimed based on the number of consecutive allocation failures. This
+ * scaling only happens for direct reclaim as it is about to attempt
+ * compaction. If compaction fails, future allocations will be deferred
+ * and reclaim avoided. On the other hand, kswapd does not take compaction
+ * deferral into account so if it scaled, it could scan excessively even
+ * though allocations are temporarily not being attempted.
   */
  static unsigned long scale_for_compaction(unsigned long pages_for_compaction,
  			struct lruvec *lruvec, struct scan_control *sc)
  {
  	struct zone *zone = lruvec_zone(lruvec);

-	if (zone->compact_order_failed <= sc->order)
+	if (zone->compact_order_failed <= sc->order &&
+	    !current_is_kswapd())
  		pages_for_compaction <<= zone->compact_defer_shift;
  	return pages_for_compaction;
  }

Yes, applying this instead of the revert fixes the issue as well.

I've applied this patch on 3.7.0-rc3 kernel - and I still see excessive CPU 
usage - mainly  after  suspend/resume

Here is just simple  kswapd backtrace from running kernel:

kswapd0         R  running task        0    30      2 0x00000000
  ffff8801331ddae8 0000000000000082 ffff880135b8a340 0000000000000008
  ffff880135b8a340 ffff8801331ddfd8 ffff8801331ddfd8 ffff8801331ddfd8
  ffff880071db8000 ffff880135b8a340 0000000000000286 ffff8801331dc000
Call Trace:
  [<ffffffff81555cd2>] preempt_schedule+0x42/0x60
  [<ffffffff81557b75>] _raw_spin_unlock+0x55/0x60
  [<ffffffff811929d1>] put_super+0x31/0x40
  [<ffffffff81192aa2>] drop_super+0x22/0x30
  [<ffffffff81193be9>] prune_super+0x149/0x1b0
  [<ffffffff81141e2a>] shrink_slab+0xba/0x510
  [<ffffffff81185baa>] ? mem_cgroup_iter+0x17a/0x2e0
  [<ffffffff81185afa>] ? mem_cgroup_iter+0xca/0x2e0
  [<ffffffff811450f9>] balance_pgdat+0x629/0x7f0
  [<ffffffff81145434>] kswapd+0x174/0x620
  [<ffffffff8106fd20>] ? __init_waitqueue_head+0x60/0x60
  [<ffffffff811452c0>] ? balance_pgdat+0x7f0/0x7f0
  [<ffffffff8106f50b>] kthread+0xdb/0xe0
  [<ffffffff8106f430>] ? kthread_create_on_node+0x140/0x140
  [<ffffffff8155fb1c>] ret_from_fork+0x7c/0xb0
  [<ffffffff8106f430>] ? kthread_create_on_node+0x140/0x140


Zdenek


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