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

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Date:	Mon, 02 May 2011 15:04:55 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	sedat.dilek@...il.com
Cc:	Chris Mason <chris.mason@...cle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG] fatal hang untarring 90GB file, possibly writeback
 related.

On Fri, 2011-04-29 at 13:02 -0500, James Bottomley wrote:
> On Fri, 2011-04-29 at 11:31 -0500, James Bottomley wrote:
> > On Fri, 2011-04-29 at 10:37 -0500, James Bottomley wrote:
> > > On Fri, 2011-04-29 at 12:23 +0200, Sedat Dilek wrote:
> > > > But as I see these RCU (CPU) stalls, the patch from [1] might be worth a try.
> > > > First, I have seen negative effects on my UP-system was when playing
> > > > with linux-next [2].
> > > > It was not clear what the origin was and the the side-effects were
> > > > somehow "bizarre".
> > > > The issue could be easily reproduced by tar-ing the kernel build-dir
> > > > to an external USB-hdd.
> > > > The issue kept RCU and TIP folks really busy.
> > > > Before stepping 4 weeks in the dark, give it a try and let me know in
> > > > case of success.
> > > 
> > > Well, it's highly unlikely because that's a 2.6.39 artifact and the bug
> > > showed up in 2.6.38 ... I tried it just in case with no effect, so we
> > > know it isn't the cause.
> > 
> > Actually, I tell a lie: it does't stop kswapd spinning on PREEMPT, but
> > it does seem to prevent non-PREEMPT from locking up totally (at least it
> > survives three back to back untar runs).
> > 
> > It's probable it alters the memory pin conditions that cause the spin,
> > so it's masking the problem rather than fixing it.
> 
> Confirmed ... it's just harder to reproduce with the hrtimers init fix.
> The problem definitely still exists (I had to load up the system more
> before doing the tar).
> 
> This time I've caught kswapd in mem_cgroup_shrink_node_zone.  sysrq-w
> doesn't complete for an unknown reason

As a follow on to this, there's a shrink_zone(0, ...) in the cgroup
path.  This causes it to scan all memory exhaustively (generating quite
a lot of work).  The comment above it implies it's some type of hack for
cgroup accounting, but reducing it to DEF_PRIORITY makes the hang go
away (verified on both 2.6.39-rc4 and 2.6.38.4).  Note that I still get
soft lockups in kswapd0, but they no longer hang the box.  I'll prepare
a patch for the next round of debate.

James


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