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Date:	Thu, 28 Apr 2011 13:30:36 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Jan Kara <jack@...e.cz>, colin.king@...onical.com,
	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>,
	linux-ext4 <linux-ext4@...r.kernel.org>, mgorman@...ell.com
Subject: Re: [BUG] fatal hang untarring 90GB file, possibly writeback
 related.

On Thu, 2011-04-28 at 18:18 +0100, Mel Gorman wrote:
> On Thu, Apr 28, 2011 at 11:56:17AM -0500, James Bottomley wrote:
> > # Events: 6K cycles
> > #
> > # Overhead      Command        Shared Object                                   Symbol
> > # ........  ...........  ...................  .......................................
> > #
> >     20.41%      kswapd0  [kernel.kallsyms]    [k] shrink_slab
> >                 |
> >                 --- shrink_slab
> >                    |          
> >                    |--99.91%-- kswapd
> >                    |          kthread
> >                    |          kernel_thread_helper
> >                     --0.09%-- [...]
> > 
> 
> Ok. I can't see how the patch "mm: vmscan: reclaim order-0 and use
> compaction instead of lumpy reclaim" is related unless we are seeing
> two problems that happen to manifest in a similar manner.
> 
> However, there were a number of changes made to dcache in particular
> for 2.6.38. Specifically thinks like dentry_kill use trylock and is
> happy to loop around if it fails to acquire anything. See things like
> this for example;

OK, so for this, I tried a 2.6.37 kernel.  It doesn't work very well,
networking is hosed for no reason I can see (probably systemd / cgroups
problems).

However, it runs enough for me to say that the tar proceeds to
completion in a non-PREEMPT kernel.  (I tried several times for good
measure).  That makes this definitely a regression of some sort, but it
doesn't definitively identify the dcache code ... it could be an ext4
bug that got introduced in 2.6.38 either.

> static void try_prune_one_dentry(struct dentry *dentry)
>         __releases(dentry->d_lock)
> {
>         struct dentry *parent;
> 
>         parent = dentry_kill(dentry, 0);
>         /*
>          * If dentry_kill returns NULL, we have nothing more to do.
>          * if it returns the same dentry, trylocks failed. In either
>          * case, just loop again.
> 
> 
> If this in combination with many inodes being locked for whatever
> reason (writeback locking them maybe?) is causing the shrinker to
> return after zero progress, it could in turn cause kswapd to enter
> into a loop for longish periods of time in shrink_slab here;
> 
>                 while (total_scan >= SHRINK_BATCH) {
>                         long this_scan = SHRINK_BATCH;
>                         int shrink_ret;
>                         int nr_before;
> 
>                         nr_before = (*shrinker->shrink)(shrinker, 0, gfp_mask);
>                         shrink_ret = (*shrinker->shrink)(shrinker, this_scan,
>                                                                 gfp_mask);
>                         if (shrink_ret == -1)
>                                 break;
>                         if (shrink_ret < nr_before)
>                                 ret += nr_before - shrink_ret;
>                         count_vm_events(SLABS_SCANNED, this_scan);
>                         total_scan -= this_scan;
> 
>                         cond_resched();
>                 }
> 
> That would explain this trace.
> 
> >      9.98%      kswapd0  [kernel.kallsyms]    [k] shrink_zone
> >                 |
> >                 --- shrink_zone
> >                    |          
> >                    |--99.46%-- kswapd
> >                    |          kthread
> >                    |          kernel_thread_helper
> >                    |          
> >                     --0.54%-- kthread
> >                               kernel_thread_helper
> > 
> >      7.70%      kswapd0  [kernel.kallsyms]    [k] kswapd
> >                 |
> >                 --- kswapd
> >                     kthread
> >                     kernel_thread_helper
> > 
> >      5.40%      kswapd0  [kernel.kallsyms]    [k] zone_watermark_ok_safe
> >                 |
> >                 --- zone_watermark_ok_safe
> >                    |          
> >                    |--72.66%-- kswapd
> >                    |          kthread
> >                    |          kernel_thread_helper
> >                    |          
> >                    |--20.88%-- sleeping_prematurely.part.12
> >                    |          kswapd
> >                    |          kthread
> >                    |          kernel_thread_helper
> >                    |          
> >                     --6.46%-- kthread
> >                               kernel_thread_helper
> > 
> 
> We are also spending an astonishing amount of time in
> sleeping_prematurely leading me to believe we are failing to balance the
> zones and are continually under the min watermark for one of the zones.
> We are never going to sleep because of this check;
> 
>                 if (total_scanned && (priority < DEF_PRIORITY - 2)) {
>                         if (has_under_min_watermark_zone)
>                                 count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
>                         else
>                                 congestion_wait(BLK_RW_ASYNC, HZ/10);
>                 }
> 
> However, I think this is a secondary effect to the failure of shrinkers
> to do their work. If slabs were being shrunk, one would expect us to
> be getting over the min watermark.
> 
> >      4.25%      kswapd0  [kernel.kallsyms]    [k] do_raw_spin_lock
> >                 |
> >                 --- do_raw_spin_lock
> >                    |          
> >                    |--77.49%-- _raw_spin_lock
> >                    |          |          
> >                    |          |--51.85%-- mb_cache_shrink_fn
> >                    |          |          shrink_slab
> >                    |          |          kswapd
> >                    |          |          kthread
> >                    |          |          kernel_thread_helper
> >                    |          |          
> >                    |           --48.15%-- mem_cgroup_soft_limit_reclaim
> >                    |                     kswapd
> >                    |                     kthread
> >                    |                     kernel_thread_helper
> >                    |          
> 
> Way hey, cgroups are also in the mix. How jolly.
> 
> Is systemd a common element of the machines hitting this bug by any
> chance?

Well, yes, the bug report is against FC15, which needs cgroups for
systemd.

> The remaining traces seem to be follow-on damage related to the three
> issues of "shrinkers are bust in some manner" causing "we are not
> getting over the min watermark" and as a side-show "we are spending lots
> of time doing something unspecified but unhelpful in cgroups".

Heh, well find a way for me to verify this: I can't turn off cgroups
because systemd then won't work and the machine won't boot ...

James


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