RE: [PATCH 1/2] btrfs: reada: limit max works count

[Zhao Lei <zhaolei@cn.fujitsu.com>]

Hi, Chris Mason

> -----Original Message-----
> From: Chris Mason [mailto:clm@fb.com]
> Sent: Friday, January 22, 2016 10:19 PM
> To: Zhao Lei <zhaolei@cn.fujitsu.com>
> Cc: linux-btrfs@vger.kernel.org
> Subject: Re: [PATCH 1/2] btrfs: reada: limit max works count
> 
> On Fri, Jan 22, 2016 at 08:25:56PM +0800, Zhao Lei wrote:
> > Hi, Chris Mason
> >
> > > -----Original Message-----
> > > From: Chris Mason [mailto:clm@fb.com]
> > > Sent: Thursday, January 21, 2016 10:15 PM
> > > To: Zhao Lei <zhaolei@cn.fujitsu.com>
> > > Cc: linux-btrfs@vger.kernel.org
> > > Subject: Re: [PATCH 1/2] btrfs: reada: limit max works count
> > >
> > > On Thu, Jan 21, 2016 at 06:06:21PM +0800, Zhao Lei wrote:
> > > > Hi, Chris Mason
> > > >
> > > > > -----Original Message-----
> > > > > From: Chris Mason [mailto:clm@fb.com]
> > > > > Sent: Thursday, January 21, 2016 1:48 AM
> > > > > To: Zhao Lei <zhaolei@cn.fujitsu.com>;
> > > > > linux-btrfs@vger.kernel.org
> > > > > Subject: Re: [PATCH 1/2] btrfs: reada: limit max works count
> > > > >
> > > > > On Wed, Jan 20, 2016 at 10:16:27AM -0500, Chris Mason wrote:
> > > > > > On Tue, Jan 12, 2016 at 03:46:26PM +0800, Zhao Lei wrote:
> > > > > > > reada create 2 works for each level of tree in recursion.
> > > > > > >
> > > > > > > In case of a tree having many levels, the number of created
> > > > > > > works is 2^level_of_tree.
> > > > > > > Actually we don't need so many works in parallel, this patch
> > > > > > > limit max works to BTRFS_MAX_MIRRORS * 2.
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > I don't think you end up calling atomic_dec() for every time
> > > > > > that
> > > > > > reada_start_machine() is called.  Also, I'd rather not have a
> > > > > > global static variable to limit the parallel workers, when we
> > > > > > have more than one FS mounted it'll end up limiting things too much.
> > > > > >
> > > > > > With this patch applied, I'm seeing deadlocks during btrfs/066.
> You
> > > > > > have to run the scrub tests as well, basically we're just
> > > > > > getting fsstress run alongside scrub.
> > > > > >
> > > > > > I'll run a few more times with it reverted to make sure, but I
> > > > > > think it's the root cause.
> > > > >
> > > > > I spoke too soon, it ended up deadlocking a few tests later.
> > > > >
> > > > In logic, even if the calculation of atomic_dec() in this patch
> > > > having bug, in worst condition, reada will works in single-thread
> > > > mode, and will not introduce deadlock.
> > > >
> > > > And by looking the backtrace in this mail, maybe it is caused by
> > > > reada_control->elems in someplace of this patchset.
> > > >
> > > > I recheck xfstests/066 in both vm and physical machine, on top of
> > > > my pull-request git today, with btrfs-progs 4.4 for many times,
> > > > but had not
> > > triggered the bug.
> > >
> > > Just running 066 alone doesn't trigger it for me.  I have to run
> > > everything from
> > > 00->066.
> > >
> > > My setup is 5 drives.  I use a script to carve them up into logical
> > > volumes, 5 for the test device and 5 for the scratch pool.  I think
> > > it should reproduce with a single drive, if you still can't trigger I'll confirm
> that.
> > >
> > > >
> > > > Could you tell me your test environment(TEST_DEV size, mount
> > > > option), and odds of fails in btrfs/066?
> > >
> > > 100% odds of failing, one time it made it up to btrfs/072.  I think
> > > more important than the drive setup is that I have all the debugging on.
> > > CONFIG_DEBUG_PAGEALLOC, spinlock debugging, mutex debugging and
> lock
> > > dep enabled.
> > >
> > Thanks for your answer.
> >
> > But unfortunately I hadn't reproduce the dead_lock in above way today...
> > Now I queued loop of above reproduce script in more nodes, and hopes
> > it can happen in this weekend.
> >
> > And by reviewing code, I found a problem which can introduce similar
> > bad result in logic, and made a patch for it.
> > [PATCH] [RFC] btrfs: reada: avoid undone reada extents in
> > btrfs_reada_wait
> >
> > Because it is only a problem in logic, but rarely happened, I only
> > confirmed no-problem after patch applied.
> >
> > Sorry for increased your works, could you apply this patch and test is
> > it works?
> 
> No problem, I'll try the patch and see if I can get a more reliable way to
> reproduce if it doesn't fix things.  Thanks!
> 
Thanks for your effective help.

I reproduced the bug in one of my node.
And I got the bug reason.

1: The background read thread in reada is not designed to complete
  all works, as above description in this mail, plus addition case in
  following.

2: For DUP, current code created 2 zones for it, and one of the zone
  is "dummy"(we only read first strip for DUP).
  And when the "dummy" zone is selected, current code ignore
  read action, just bypass and do a cleanup.
  Current code just return without re-select zone in this case to make
  logic easy, and it make code likely to exit reada thread.
  So, in DUP case, more background thread exit before all works done.
  It is why btrfs/066 always hang in DUP profile.

3: This problem exist in old code too, but rarely happened, my patchset
  trigger the problem because:
  a. Limited background thread number
    PATCH: btrfs: reada: limit max works count
    In old code, there are more background threads, and if one of them exit,
    remain threads will continue remain extents.
  b. reduce thread lift time
    PATCH: btrfs: reada: Avoid many times of empty loop
    The lift time of thread is reduced, and make the no-thread window large.

Fix: 
  We have following solution for this problem:
  a. Not add above dummy zone for DUP
    It will reduce the happen odds of the problem, but because
    "device workload limit(MAX_IN_FLIGHT)" and "total reads limit"
    in code, so the problem will still exist in very small case.
  b. let the reada background thread do all works before exit.
    It conflict with the limit design in [a].
  c. Check to ensure we have at least one thread in btrfs_reada_wait()
    It can fix the problem completely.
  So I will fix the problem by way "c", based on:
  [RFC] btrfs: reada: avoid undone reada extents in btrfs_reada_wait
  With some enhancement.

I'll make the fix and test it.

Thanks
Zhaolei

> -chris
>