Re: 4.8.8 kernel trigger OOM killer repeatedly when I have lots of RAM that should be free

[Marc MERLIN]

Howdy,

Well, sadly, the problem is more or less back is 4.11.0. The system doesn't really 
crash but it goes into an infinite loop with
[34776.826800] BUG: workqueue lockup - pool cpus=6 node=0 flags=0x0 nice=0 stuck for 33s!
More logs: https://pastebin.com/YqE4riw0

(I upgraded from 4.8 with custom patches you gave me, and went to 4.11.0

gargamel:~# cat /proc/sys/vm/dirty_ratio
2
gargamel:~# cat /proc/sys/vm/dirty_background_ratio
1
gargamel:~# free
             total       used       free     shared    buffers     cached
Mem:      24392600   16362660    8029940          0       8884   13739000
-/+ buffers/cache:    2614776   21777824
Swap:     15616764          0   15616764

And yet, I was doing a btrfs check repair on a busy filesystem, within 40mn or so,
it triggered the workqueue lockup.

gargamel:~# grep CONFIG_COMPACTION /boot/config-4.11.0-amd64-preempt-sysrq-20170406 
CONFIG_COMPACTION=y

kernel config file: https://pastebin.com/7Tajse6L

To be fair, I didn't try to run btrfs check on 4.8 and now I'm busy
trying to recover a filesystem that apparently got corrupted by a bad
SAS driver in 4.8 which caused a lot of I/O errors and corruption.
This is just to say that btrfs on top of dmcrypt on top of bcache may
have been enough layers to hang on btrfs check on 4.8 too, but I can't
really go back to check right now due to the driver corruption issues.

Any idea what I should do next?

Thanks,
Marc

On Tue, Nov 29, 2016 at 03:01:35PM -0800, Marc MERLIN wrote:
> On Tue, Nov 29, 2016 at 09:40:19AM -0800, Marc MERLIN wrote:
> > Thanks for the reply and suggestions.
> > 
> > On Tue, Nov 29, 2016 at 09:07:03AM -0800, Linus Torvalds wrote:
> > > On Tue, Nov 29, 2016 at 8:34 AM, Marc MERLIN <marc@xxxxxxxxxxx> wrote:
> > > > Now, to be fair, this is not a new problem, it's just varying degrees of
> > > > bad and usually only happens when I do a lot of I/O with btrfs.
> > > 
> > > One situation where I've seen something like this happen is
> > > 
> > >  (a) lots and lots of dirty data queued up
> > >  (b) horribly slow storage
> > 
> > In my case, it is a 5x 4TB HDD with 
> > software raid 5 < bcache < dmcrypt < btrfs
> > bcache is currently half disabled (as in I removed the actual cache) or
> > too many bcache requests pile up, and the kernel dies when too many
> > workqueues have piled up.
> > I'm just kind of worried that since I'm going through 4 subsystems
> > before my data can hit disk, that's a lot of memory allocations and
> > places where data can accumulate and cause bottlenecks if the next
> > subsystem isn't as fast.
> > 
> > But this shouldn't be "horribly slow", should it? (it does copy a few
> > terabytes per day, not fast, but not horrible, about 30MB/s or so)
> > 
> > > Sadly, our defaults for "how much dirty data do we allow" are somewhat
> > > buggered. The global defaults are in "percent of memory", and are
> > > generally _much_ too high for big-memory machines:
> > > 
> > >     [torvalds@i7 linux]$ cat /proc/sys/vm/dirty_ratio
> > >     20
> > >     [torvalds@i7 linux]$ cat /proc/sys/vm/dirty_background_ratio
> > >     10
> > 
> > I can confirm I have the same.
> > 
> > > says that it only starts really throttling writes when you hit 20% of
> > > all memory used. You don't say how much memory you have in that
> > > machine, but if it's the same one you talked about earlier, it was
> > > 24GB. So you can have 4GB of dirty data waiting to be flushed out.
> > 
> > Correct, 24GB and 4GB.
> > 
> > > And we *try* to do this per-device backing-dev congestion thing to
> > > make things work better, but it generally seems to not work very well.
> > > Possibly because of inconsistent write speeds (ie _sometimes_ the SSD
> > > does really well, and we want to open up, and then it shuts down).
> > > 
> > > One thing you can try is to just make the global limits much lower. As in
> > > 
> > >    echo 2 > /proc/sys/vm/dirty_ratio
> > >    echo 1 > /proc/sys/vm/dirty_background_ratio
> > 
> > I will give that a shot, thank you.
> 
> And, after 5H of copying, not a single hang, or USB disconnect, or anything.
> Obviously this seems to point to other problems in the code, and I have no
> idea which layer is a culprit here, but reducing the buffers absolutely
> helped a lot.

-- 
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Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
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