Re: [BUG] fatal hang untarring 90GB file, possibly writeback related.

[Mel Gorman]

On Thu, Apr 28, 2011 at 09:25:14AM -0500, James Bottomley wrote:
> On Thu, 2011-04-28 at 15:07 +0100, Mel Gorman wrote:
> > On Thu, Apr 28, 2011 at 03:52:28PM +0200, Jan Kara wrote:
> > > On Thu 28-04-11 12:36:30, Colin Ian King wrote:
> > > > One more data point to add, I've been looking at an identical issue when
> > > > copying large amounts of data.  I bisected this - and the lockups occur
> > > > with commit 
> > > > 3e7d344970673c5334cf7b5bb27c8c0942b06126 - before that I don't see the
> > > > issue. With this commit, my file copy test locks up after ~8-10
> > > > iterations, before this commit I can copy > 100 times and don't see the
> > > > lockup.
> > >   Adding Mel to CC, I guess he'll be interested. Mel, it seems this commit
> > > of yours causes kswapd on non-preempt kernels spin for a *long* time...
> > > 
> > 
> > I'm still thinking about the traces which do not point the finger
> > directly at compaction per-se but it's possible that the change means
> > kswapd is not reclaiming like it should be.
> > 
> > To test this theory, does applying
> > [d527caf2: mm: compaction: prevent kswapd compacting memory to reduce
> > CPU usage] help?
> 
> I can answer definitively no to this.  The upstream kernel I reproduced
> this on has that patch included.
> 

So it is.

Another consequence of this patch is that when high order allocations
are in progress (is the test case fork heavy in any way for
example? alternatively, it might be something in the storage stack
that requires high-order allocs) we are no longer necessarily going
to sleep because of should_reclaim_continue() check. This could
explain kswapd-at-99% but would only apply if CONFIG_COMPACTION is
set (does unsetting CONFIG_COMPACTION help). If the bug only triggers
for CONFIG_COMPACTION, does the following *untested* patch help any?

(as a warning, I'm offline Friday until Tuesday morning. I'll try
check mail over the weekend but it's unlikely I'll find a terminal
or be allowed to use it without an ass kicking)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 148c6e6..c74a501 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1842,15 +1842,22 @@ static inline bool should_continue_reclaim(struct zone *zone,
 		return false;
 
 	/*
-	 * If we failed to reclaim and have scanned the full list, stop.
-	 * NOTE: Checking just nr_reclaimed would exit reclaim/compaction far
-	 *       faster but obviously would be less likely to succeed
-	 *       allocation. If this is desirable, use GFP_REPEAT to decide
-	 *       if both reclaimed and scanned should be checked or just
-	 *       reclaimed
+	 * For direct reclaimers
+	 *   If we failed to reclaim and have scanned the full list, stop.
+	 *   The caller will check congestion and sleep if necessary until
+	 *   some IO completes.
+	 * For kswapd
+	 *   Check just nr_reclaimed. If we are failing to reclaim, we
+	 *   want to stop this reclaim loop, increase the priority and
+	 *   go to sleep if necessary to allow IO a change to complete.
+	 *   This avoids kswapd going into a busy loop in shrink_zone()
 	 */
-	if (!nr_reclaimed && !nr_scanned)
-		return false;
+	if (!nr_reclaimed) {
+		if (current_is_kswapd())
+			return false;
+		else if (!nr_scanned)
+			return false;
+	}
 
 	/*
 	 * If we have not reclaimed enough pages for compaction and the
@@ -1924,8 +1931,13 @@ restart:
 
 	/* reclaim/compaction might need reclaim to continue */
 	if (should_continue_reclaim(zone, nr_reclaimed,
-					sc->nr_scanned - nr_scanned, sc))
+					sc->nr_scanned - nr_scanned, sc)) {
+		/* Throttle direct reclaimers if congested */
+		if (!current_is_kswapd())
+			wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
+
 		goto restart;
+	}
 
 	throttle_vm_writeout(sc->gfp_mask);
 }
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