Re: [RFC] xfs: Flush iclog containing XLOG_COMMIT_TRANS before waiting for log space

[Chandan Rajendra <chandan@linux.ibm.com>]

On Friday, August 23, 2019 7:08 PM Chandan Rajendra wrote:
> On Friday, August 23, 2019 6:38 PM Brian Foster wrote:
> > On Fri, Aug 23, 2019 at 10:06:36AM +1000, Dave Chinner wrote:
> > > On Thu, Aug 22, 2019 at 12:34:46PM -0400, Brian Foster wrote:
> > > > On Thu, Aug 22, 2019 at 08:18:34AM +1000, Dave Chinner wrote:
> > > > > On Wed, Aug 21, 2019 at 04:34:48PM +0530, Chandan Rajendra wrote:
> > > > > > The following call trace is seen when executing generic/530 on a ppc64le
> > > > > > machine,
> > > > > > 
> > > > > > INFO: task mount:7722 blocked for more than 122 seconds.
> > > > > >       Not tainted 5.3.0-rc1-next-20190723-00001-g1867922e5cbf-dirty #6
> > > > > 
> > > > > can you reproduce this on 5.3-rc5? There were bugs in log recovery
> > > > > IO in -rc1 that could result in things going wrong...
> > > > > 
> > > > > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > > > > mount           D 8448  7722   7490 0x00040008
> > > > > > Call Trace:
> > > > > > [c000000629343210] [0000000000000001] 0x1 (unreliable)
> > > > > > [c0000006293433f0] [c000000000021acc] __switch_to+0x2ac/0x490
> > > > > > [c000000629343450] [c000000000fbbbf4] __schedule+0x394/0xb50
> > > > > > [c000000629343510] [c000000000fbc3f4] schedule+0x44/0xf0
> > > > > > [c000000629343540] [c0000000007623b4] xlog_grant_head_wait+0x84/0x420
> > > > > > [c0000006293435b0] [c000000000762828] xlog_grant_head_check+0xd8/0x1e0
> > > > > > [c000000629343600] [c000000000762f6c] xfs_log_reserve+0x26c/0x310
> > > > > > [c000000629343690] [c00000000075defc] xfs_trans_reserve+0x28c/0x3e0
> > > > > > [c0000006293436e0] [c0000000007606ac] xfs_trans_alloc+0xfc/0x2f0
> > > > > > [c000000629343780] [c000000000749ca8] xfs_inactive_ifree+0x248/0x2a0
> > > > > > [c000000629343810] [c000000000749e58] xfs_inactive+0x158/0x300
> > > > > > [c000000629343850] [c000000000758554] xfs_fs_destroy_inode+0x104/0x3f0
> > > > > > [c000000629343890] [c00000000046850c] destroy_inode+0x6c/0xc0
> > > > > > [c0000006293438c0] [c00000000074c748] xfs_irele+0x168/0x1d0
> > > > > > [c000000629343900] [c000000000778c78] xlog_recover_process_one_iunlink+0x118/0x1e0
> > > > > > [c000000629343960] [c000000000778e10] xlog_recover_process_iunlinks+0xd0/0x130
> > > > > > [c0000006293439b0] [c000000000782408] xlog_recover_finish+0x58/0x130
> > > > > > [c000000629343a20] [c000000000763818] xfs_log_mount_finish+0xa8/0x1d0
> > > > > > [c000000629343a60] [c000000000750908] xfs_mountfs+0x6e8/0x9e0
> > > > > > [c000000629343b20] [c00000000075a210] xfs_fs_fill_super+0x5a0/0x7c0
> > > > > > [c000000629343bc0] [c00000000043e7fc] mount_bdev+0x25c/0x2a0
> > > > > > [c000000629343c60] [c000000000757c48] xfs_fs_mount+0x28/0x40
> > > > > > [c000000629343c80] [c0000000004956cc] legacy_get_tree+0x4c/0xb0
> > > > > > [c000000629343cb0] [c00000000043d690] vfs_get_tree+0x50/0x160
> > > > > > [c000000629343d30] [c0000000004775d4] do_mount+0xa14/0xc20
> > > > > > [c000000629343db0] [c000000000477d48] ksys_mount+0xc8/0x180
> > > > > > [c000000629343e00] [c000000000477e20] sys_mount+0x20/0x30
> > > > > > [c000000629343e20] [c00000000000b864] system_call+0x5c/0x70
> > > > > > 
> > > > > > i.e. the mount task gets hung indefinitely due to the following sequence
> > > > > > of events,
> > > > > > 
> > > > > > 1. Test creates lots of unlinked temp files and then shutsdown the
> > > > > >    filesystem.
> > > > > > 2. During mount, a transaction started in the context of processing
> > > > > >    unlinked inode list causes several iclogs to be filled up. All but
> > > > > >    the last one is submitted for I/O.
> > > > > > 3. After writing XLOG_COMMIT_TRANS record into the iclog, we will have
> > > > > >    18532 bytes of free space in the last iclog of the transaction which is
> > > > > >    greater than 2*sizeof(xlog_op_header_t). Hence
> > > > > >    xlog_state_get_iclog_space() does not switch over to using a newer iclog.
> > > > > > 4. Meanwhile, the endio code processing iclogs of the transaction do not
> > > > > >    insert items into the AIL since the iclog containing XLOG_COMMIT_TRANS
> > > > > >    hasn't been submitted for I/O yet. Hence a major part of the on-disk
> > > > > >    log cannot be freed yet.
> > > > > 
> > > > > So all those items are still pinned in memory.
> > > > > 
> > > > > > 5. A new request for log space (via xfs_log_reserve()) will now wait
> > > > > >    indefinitely for on-disk log space to be freed.
> > > > > 
> > > > > Because nothing has issued a xfs_log_force() for write the iclog to
> > > > > disk, unpin the objects that it pins in memory, and allow the tail
> > > > > to be moved forwards.
> > > > > 
> > > > > The xfsaild normally takes care of thisi - it gets pushed byt the
> > > > > log reserve when there's not enough space to in the log for the
> > > > > transaction before transaction reserve goes to sleep in
> > > > > xlog_grant_head_wait(). The AIL pushing code is then responsible for
> > > > > making sure log space is eventually freed. It will issue log forces
> > > > > if it isn't making progress and so this problem shouldn't occur.
> > > > > 
> > > > > So, why has it occurred?
> > > > > 
> > > > > The xfsaild kthread should be running at this point, so if it was
> > > > > pushed it should be trying to empty the journal to move the tail
> > > > > forward. Why hasn't it issue a log force?
> > > > > 
> > > > > 
> > > > > > To fix this issue, before waiting for log space to be freed, this commit
> > > > > > now submits xlog->l_iclog for write I/O if iclog->ic_state is
> > > > > > XLOG_STATE_ACTIVE and iclog has metadata written into it. This causes
> > > > > > AIL list to be populated and a later call to xlog_grant_push_ail() will
> > > > > > free up the on-disk log space.
> > > > > 
> > > > > hmmm.
> > > > > 
> > > > > > Signed-off-by: Chandan Rajendra <chandanrlinux@gmail.com>
> > > > > > ---
> > > > > >  fs/xfs/xfs_log.c | 21 +++++++++++++++++++++
> > > > > >  1 file changed, 21 insertions(+)
> > > > > > 
> > > > > > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> > > > > > index 00e9f5c388d3..dc785a6b9f47 100644
> > > > > > --- a/fs/xfs/xfs_log.c
> > > > > > +++ b/fs/xfs/xfs_log.c
> > > > > > @@ -236,11 +236,32 @@ xlog_grant_head_wait(
> > > > > >  	int			need_bytes) __releases(&head->lock)
> > > > > >  					    __acquires(&head->lock)
> > > > > >  {
> > > > > > +	struct xlog_in_core	*iclog;
> > > > > > +
> > > > > >  	list_add_tail(&tic->t_queue, &head->waiters);
> > > > > >  
> > > > > >  	do {
> > > > > >  		if (XLOG_FORCED_SHUTDOWN(log))
> > > > > >  			goto shutdown;
> > > > > > +
> > > > > > +		if (xfs_ail_min(log->l_ailp) == NULL) {
> > > > > 
> > > > > This is indicative of the situation. If the AIL is empty, and the
> > > > > log does not have room for an entire transaction reservation, then
> > > > > we need to be issuing synchronous transactions in recovery until
> > > > > such time the AIL pushing can actually function correctly to
> > > > > guarantee forwards progress for async transaction processing.
> > > > > 
> > > > 
> > > > Hmm, I don't think that addresses the fundamental problem. This
> > > > phenomenon doesn't require log recovery. The same scenario can present
> > > > itself after a clean mount or from an idle fs. I think the scenario that
> > > > plays out here, at a high level, is as follows:
> > > > 
> > > 
> > >   - mount
> > >   - [log recovery]
> > >   - xfs_log_mount_finish
> > >     - calls xfs_log_work_queue()
> > > 
> > > > - Heavy transaction workload commences. This continuously acquires log
> > > >   reservation and transfers it to the CIL as transactions commit.
> > > > - The CIL context grows until we cross the background threshold, at
> > > >   which point we schedule a background push.
> > > > - Background CIL push cycles the current context into the log via the
> > > >   iclog buffers. The commit record stays around in-core because the last
> > > >   iclog used for the CIL checkpoint isn't full. Hence, none of the
> > > >   associated log items make it into the AIL and the background CIL push
> > > >   had no effect with respect to freeing log reservation.
> > > > - The same transaction workload is still running and filling up the next
> > > >   CIL context. If we run out of log reservation before a second
> > > >   background CIL push comes along, we're basically stuck waiting on
> > > >   somebody to force the log.
> > > 
> > > - every 30s xfs_log_worker() runs, sees the log dirty, triggers
> > >   a log force. pending commit is flushed to log, dirty objects get
> > >   moved to AIL, then xfs_log_worker() pushes on the AIL to do
> > >   periodic background metadata writeback.
> > > 
> > > > The things that prevent this at normal runtime are timely execution of
> > > > background CIL pushes and the background log worker. If for some reason
> > > > the background CIL push is not timely enough that we consume all log
> > > > reservation before two background CIL pushes occur from the time the
> > > > racing workload begins (i.e. starting from an idle system such that the
> > > > AIL is empty), then we're stuck waiting on the background log worker to
> > > > force the log from the first background CIL push, populate the AIL and
> > > > get things moving again.
> > > 
> > > Right, this does not deadlock - it might pause for a short while
> > > while waiting for the log worker to run and issue a log force. I
> > > have never actually seen it happen in all my years of "mkfs; mount;
> > > fsmark" testing that places a /massive/ metadata modification
> > > workload on a pristine, newly mounted filesystems....
> > > 
> > 
> > Neither have I. That's why I'm asking for more details on how this
> > triggers. :) Whatever factors contribute to this particular instance may
> > or may not apply outside of log recovery context.
> > 
> > > As it is, we've always used the log worker as a watchdog in this
> > > way. The fact is that we have very few situations left in the code
> > > where it needs to act as a watchdog - delayed logging actually
> > > negated the vast majority of problems that required the periodic log
> > > force to get out of trouble because individual transactions no
> > > longer needed to wait on iclog space to make progress...
> > > 
> > 
> > Yes, the description I wrote up already explains that the background log
> > worker changes the symptom from a deadlock to a stall. The defaults
> > allow for a stall of up to 30s, but custom configuration allows for a
> > stall of up to 2 hours.
> > 
> > > > IOW, the same essential problem is reproducible outside of log
> > > > recovery in the form of stalls as opposed to deadlocks via an
> > > > artificial background CIL push delay (i.e., think workqueue or
> > > > xc_cil_ctx lock starvation) and an elevated xfssyncd_centisecs.
> > > > We aren't stuck forever because the background log worker will run
> > > > eventually, but it could certainly be a dead stall of minutes
> > > > before that occurs.
> > > 
> > > I don't think addressing it in xlog_grant_head_wait() fixes the
> > > problem fully, either.  If no other transaction comes in, then the
> > > ones already blocked (because the AIL was not empty when they tried
> > > to reserve space) will end up still blocked because nothing has
> > > kicked the code in the transaction reservation code. So putting the
> > > log force into the grant head wait code is not sufficient by itself.
> > > 
> > > > I think this
> > > > could still be addressed at transaction commit or reservation time, but
> > > > I think the logic needs to be more generic and based on log reservation
> > > > pressure rather than the context from which this particular test happens
> > > > to reproduce.
> > > 
> > > Log reservation pressure is what xlog_grant_push_ail() detects and
> > > that pressure is transferred to the AIL pushing code to clean dirty
> > > log items and move the tail of the log forward. It's right there
> > > where Chandan added the log force. :)
> > > 
> > > IOWs, xlog_grant_push_ail() tells the xfsaild how much log space to
> > > make available. If the log is full, then xlog_grant_push_ail() will
> > > already be telling the AIL to push and will be waking it up.
> > > However, the aild will see the AIL empty and go right back to sleep.
> > > That's likely the runtime problem here - the mechanism that pushes
> > > the log tail forwards is not realising that the log needs pushing
> > > via a log force.
> > > 
> > > IOWs, I suspect the xfsaild is the right place to take the action,
> > > because AIL pushing is triggered by much more than just log
> > > reservations. It gets kicked by memory reclaim, the log worker, new
> > > transactions, etc and so if a transaction doesn't kick it when it
> > > gest stuck like this, something else will.
> > > 
> > 
> > Perhaps, I could see that as an option too. TBH, it's not clear to me
> > where the best place to fix this is. I want to know more about the
> > reproducer first. My comment above was just that I'm not convinced the
> > previously proposed logic to force based on log recovery context is
> > correct.
> > 
> > > > If this is all on the right track, I'm still curious if/how you're
> > > > getting into a situation where all log reservation is held up in the CIL
> > > > before a couple background pushes occur.
> > > 
> > > I'd guess that it could be reproduced via a single CPU machine and
> > > non-preempt kernel. We've already replayed all the unlink
> > > transactions, so the buffer/inode caches are fully primed. If all
> > > unlink removal transactions the buffer/inode cache, then it won't
> > > block anywhere and will never yield the CPU. Hence the CIL push
> > > kworker thread doesn't get to run before the unlinks run the log out
> > > of space.
> > > 
> > 
> > Yeah, I wouldn't expect that for a ppc64 system, but I suppose this
> > could be a VM or something too. Chandan?
> >
> 
> Indeed, This is a KVM guest on a ppc64le machine. The guest has 8 vcpus
> allocated to it. Also, Linux on ppc64le is by default configured to be
> non-preemptable.
> 
> Btw, the issue is still re-creatable after adding,
> 
> +               if ((log->l_flags & XLOG_RECOVERY_NEEDED)
> +                       && !xfs_ail_min(tp->t_mountp->m_ail))
> +                       tp->t_flags |= XFS_TRANS_SYNC;
> 
> ... to xfs_trans_reserve(). I didn't get a chance to debug it yet. I will try
> to find the root cause now.
> 

Dave, With the above changes made in xfs_trans_reserve(), mount task is
deadlocking due to the following,
1. With synchronous transactions, __xfs_trans_commit() now causes iclogs to be
flushed to the disk and hence log items to be ultimately moved to AIL.
2. xfsaild task is woken up, which acts in items on AIL.
3. After some time, we stop issuing synchronous transactions because AIL has
   log items in its list and hence !xfs_ail_min(tp->t_mountp->m_ail) evaluates to
   false. In xfsaild_push(), "XFS_LSN_CMP(lip->li_lsn, target) <= 0"
   evaluates to false on the first iteration of the while loop. This means we
   have a log item whose LSN is larger than xfs_ail->ail_target at the
   beginning of the AIL.
4. Now unlinked inode list processing blocks waiting for space reservation on
   the log.

I will debug and get more details as to how AIL ended up in this situation.

-- 
chandan