On Sun, 2011-05-15 at 19:27 +0900, KOSAKI Motohiro wrote:

(2011/05/13 23:03), Mel Gorman wrote:

quoted

Under constant allocation pressure, kswapd can be in the situation where
sleeping_prematurely() will always return true even if kswapd has been
running a long time. Check if kswapd needs to be scheduled.

Signed-off-by: Mel Gorman<mgorman@suse.de>
---
  mm/vmscan.c |    4 ++++
  1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index af24d1e..4d24828 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
  	unsigned long balanced = 0;
  	bool all_zones_ok = true;

+	/* If kswapd has been running too long, just sleep */
+	if (need_resched())
+		return false;
+

Hmm... I don't like this patch so much. because this code does

- don't sleep if kswapd got context switch at shrink_inactive_list

This isn't entirely true:  need_resched() will be false, so we'll follow
the normal path for determining whether to sleep or not, in effect
leaving the current behaviour unchanged.

- sleep if kswapd didn't

This also isn't entirely true: whether need_resched() is true at this
point depends on a whole lot more that whether we did a context switch
in shrink_inactive. It mostly depends on how long we've been running
without giving up the CPU.  Generally that will mean we've been round
the shrinker loop hundreds to thousands of times without sleeping.

It seems to be semi random behavior.

Well, we have to do something.  Chris Mason first suspected the hang was
a kswapd rescheduling problem a while ago.  We tried putting
cond_rescheds() in several places in the vmscan code, but to no avail.
The need_resched() in sleeping_prematurely() seems to be about the best
option.  The other option might be just to put a cond_resched() in
kswapd_try_to_sleep(), but that will really have about the same effect.

James