Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long,allow it to sleep

From: KOSAKI Motohiro
Date: Wed May 18 2011 - 02:06:31 EST

While it appears unlikely, there are bad conditions which can result

in cond_resched() being avoided.

Every reclaim priority decreasing or every shrink_zone() calling makes more
fine grained preemption. I think.

It could be.
But in direct reclaim case, I have a concern about losing pages
reclaimed to other tasks by preemption.

Nope, I proposed to add cond_resched() into balance_pgdat().

Hmm,, anyway, we also needs test.
Hmm,, how long should we bother them(Colins and James)?
First of all, Let's fix one just between us and ask test to them and
send the last patch to akpm.

1. shrink_slab
2. right after balance_pgdat
3. shrink_zone
4. reclaim priority decreasing routine.

Now, I vote 1) and 2).


I think following patch makes enough preemption.


From e7d88be1916184ea7c93a6f2746b15c7a32d1973 Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Date: Wed, 18 May 2011 15:00:39 +0900
Subject: [PATCH] vmscan: balance_pgdat() call cond_resched() unconditionally

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: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Colin King <colin.king@xxxxxxxxxxxxx>
Cc: Minchan Kim <>
mm/vmscan.c | 3 +--
1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 19e179b..87c88fd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2449,6 +2449,7 @@ loop_again:
sc.nr_reclaimed += reclaim_state->reclaimed_slab;
total_scanned += sc.nr_scanned;

+ cond_resched();
if (zone->all_unreclaimable)
if (nr_slab == 0 &&
@@ -2518,8 +2519,6 @@ out:
* for the node to be balanced
if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))) {
- cond_resched();


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