Re: Processes spinning forever, apparently in lock_timer_base()?

[Chuck Ebbert <cebbert@redhat.com>]

On 09/20/2007 05:29 PM, Andrew Morton wrote:
> On Thu, 20 Sep 2007 17:07:15 -0400
> Chuck Ebbert <cebbert@redhat.com> wrote:
> 
>> On 08/09/2007 12:55 PM, Andrew Morton wrote:
>>> On Thu, 9 Aug 2007 11:59:43 +0200 Matthias Hensler <matthias@wspse.de> wrote:
>>>
>>>> On Sat, Aug 04, 2007 at 10:44:26AM +0200, Matthias Hensler wrote:
>>>>> On Fri, Aug 03, 2007 at 11:34:07AM -0700, Andrew Morton wrote:
>>>>> [...]
>>>>> I am also willing to try the patch posted by Richard.
>>>> I want to give some update here:
>>>>
>>>> 1. We finally hit the problem on a third system, with a total different
>>>>    setup and hardware. However, again high I/O load caused the problem
>>>>    and the affected filesystems were mounted with noatime.
>>>>
>>>> 2. I installed a recompiled kernel with just the two line patch from
>>>>    Richard Kennedy (http://lkml.org/lkml/2007/8/2/89). That system has 5
>>>>    days uptime now and counting. I believe the patch fixed the problem.
>>>>    However, I will continue running "vmstat 1" and the endless loop of
>>>>    "cat /proc/meminfo", just in case I am wrong.
>>>>
>>> Did we ever see the /proc/meminfo and /proc/vmstat output during the stall?
>>>
>>> If Richard's patch has indeed fixed it then this confirms that we're seeing
>>> contention over the dirty-memory limits.  Richard's patch isn't really the
>>> right one because it allows unlimited dirty-memory windup in some situations
>>> (large number of disks with small writes, or when we perform queue congestion
>>> avoidance).
>>>
>>> As you're seeing this happening when multiple disks are being written to it is
>>> possible that the per-device-dirty-threshold patches which recently went into
>>> -mm (and which appear to have a bug) will fix it.
>>>
>>> But I worry that the stall appears to persist *forever*.  That would indicate
>>> that we have a dirty-memory accounting leak, or that for some reason the
>>> system has decided to stop doing writeback to one or more queues (might be
>>> caused by an error in a lower-level driver's queue congestion state management).
>>>
>>> If it is the latter, then it could be that running "sync" will clear the
>>> problem.  Temporarily, at least.  Because sync will ignore the queue congestion
>>> state.
>>>
>> This is still a problem for people, and no fix is in sight until 2.6.24.
> 
> Any bugzilla urls or anything like that?

https://bugzilla.redhat.com/show_bug.cgi?id=249563

> 
>> Can we get some kind of band-aid, like making the endless 'for' loop in
>> balance_dirty_pages() terminate after some number of iterations? Clearly
>> if we haven't written "write_chunk" pages after a few tries, *and* we
>> haven't encountered congestion, there's no point in trying forever...
> 
> Did my above questions get looked at?
> 
> Is anyone able to reproduce this?
> 
> Do we have a clue what's happening?

There are a ton of dirty pages for one disk, and zero or close to zero dirty
for a different one. Kernel spins forever trying to write some arbitrary
minimum amount of data ("write_chunk" pages) to the second disk...

> 
> Is that function just spinning around, failing to start writeout against
> any pages at all?  If so, how come?

Yes, it spins forever. Just removing the "noatime" mount option for the
second disk generates enough dirty data to keep the system functional.
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