Re: [PATCH] xfs: Wake CIL push waiters more reliably

[Brian Foster <bfoster@redhat.com>]

On Fri, Jan 08, 2021 at 11:56:57AM -0500, Brian Foster wrote:
> On Fri, Jan 08, 2021 at 08:54:44AM +1100, Dave Chinner wrote:
> > On Mon, Jan 04, 2021 at 11:23:53AM -0500, Brian Foster wrote:
> > > On Thu, Dec 31, 2020 at 09:16:11AM +1100, Dave Chinner wrote:
> > > > On Wed, Dec 30, 2020 at 12:56:27AM +0100, Donald Buczek wrote:
> > > > > If the value goes below the limit while some threads are
> > > > > already waiting but before the push worker gets to it, these threads are
> > > > > not woken.
> > > > > 
> > > > > Always wake all CIL push waiters. Test with waitqueue_active() as an
> > > > > optimization. This is possible, because we hold the xc_push_lock
> > > > > spinlock, which prevents additions to the waitqueue.
> > > > > 
> > > > > Signed-off-by: Donald Buczek <buczek@molgen.mpg.de>
> > > > > ---
> > > > >  fs/xfs/xfs_log_cil.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> > > > > index b0ef071b3cb5..d620de8e217c 100644
> > > > > --- a/fs/xfs/xfs_log_cil.c
> > > > > +++ b/fs/xfs/xfs_log_cil.c
> > > > > @@ -670,7 +670,7 @@ xlog_cil_push_work(
> > > > >  	/*
> > > > >  	 * Wake up any background push waiters now this context is being pushed.
> > > > >  	 */
> > > > > -	if (ctx->space_used >= XLOG_CIL_BLOCKING_SPACE_LIMIT(log))
> > > > > +	if (waitqueue_active(&cil->xc_push_wait))
> > > > >  		wake_up_all(&cil->xc_push_wait);
> > > > 
> > > > That just smells wrong to me. It *might* be correct, but this
> > > > condition should pair with the sleep condition, as space used by a
> > > > CIL context should never actually decrease....
> > > > 
> > > 
> > > ... but I'm a little confused by this assertion. The shadow buffer
> > > allocation code refers to the possibility of shadow buffers falling out
> > > that are smaller than currently allocated buffers. Further, the
> > > _insert_format_items() code appears to explicitly optimize for this
> > > possibility by reusing the active buffer, subtracting the old size/count
> > > values from the diff variables and then reformatting the latest
> > > (presumably smaller) item to the lv.
> > 
> > Individual items might shrink, but the overall transaction should
> > grow. Think of a extent to btree conversion of an inode fork. THe
> > data in the inode fork decreases from a list of extents to a btree
> > root block pointer, so the inode item shrinks. But then we add a new
> > btree root block that contains all the extents + the btree block
> > header, and it gets rounded up to ithe 128 byte buffer logging chunk
> > size.
> > 
> > IOWs, while the inode item has decreased in size, the overall
> > space consumed by the transaction has gone up and so the CIL ctx
> > used_space should increase. Hence we can't just look at individual
> > log items and whether they have decreased in size - we have to look
> > at all the items in the transaction to understand how the space used
> > in that transaction has changed. i.e. it's the aggregation of all
> > items in the transaction that matter here, not so much the
> > individual items.
> > 
> 
> Ok, that makes more sense...
> 
> > > Of course this could just be implementation detail. I haven't dug into
> > > the details in the remainder of this thread and I don't have specific
> > > examples off the top of my head, but perhaps based on the ability of
> > > various structures to change formats and the ability of log vectors to
> > > shrink in size, shouldn't we expect the possibility of a CIL context to
> > > shrink in size as well? Just from poking around the CIL it seems like
> > > the surrounding code supports it (xlog_cil_insert_items() checks len > 0
> > > for recalculating split res as well)...
> > 
> > Yes, there may be situations where it decreases. It may be this is
> > fine, but the assumption *I've made* in lots of the CIL push code is
> > that ctx->used_space rarely, if ever, will go backwards.
> > 
> 
> ... and rarely seems a bit more pragmatic than never.
> 

FWIW, a cursory look at the inode size/format code (motivated by
Donald's recent log dump that appears to show inode log items changing
size) suggested that a simple local format size change might be enough
to cause this condition on an item. A subsequent test to create and
immediately remove a file from an otherwise empty directory triggers a
tracepoint I injected in xlog_cil_insert_items() to detect a negative
transaction delta. As expected, the absolute value of the delta does
seem to increase with a larger filename. This also produces a negative
iovec delta, fwiw. E.g.:

# touch `for i in $(seq 0 63); do echo -n a; done`
# rm -f `for i in $(seq 0 63); do echo -n a; done`
#

rm-9265    [001] ....  4660.177806: xfs_log_commit_cil: 409: len -72 diff_iovecs 0
rm-9265    [001] .N.1  4660.177913: xfs_log_commit_cil: 419: len -72 diff_iovecs 0
rm-9265    [001] ....  4660.178313: xfs_log_commit_cil: 409: len -52 diff_iovecs -1
rm-9265    [001] ...1  4660.178336: xfs_log_commit_cil: 419: len -64 diff_iovecs -1

... and this only seems to occur when the unlink occurs before the CIL
has been checkpointed and pushed out the inode (i.e. a freeze/unfreeze
cycle prevents it).

I've not dug into the transaction details and have no idea if this is
the variant that Donald reproduces; it wouldn't surprise me a ton if
there were various others. This is pretty straightforward, however, and
shows the negative item delta carry through the transaction. IMO, that
seems to justify a throttling fix...

Brian

> > e.g. we run the first transaction into the CIL, it steals the sapce
> > needed for the cil checkpoint headers for the transaciton. Then if
> > the space returned by the item formatting is negative (because it is
> > in the AIL and being relogged), the CIL checkpoint now doesn't have
> > the space reserved it needs to run a checkpoint. That transaction is
> > a sync transaction, so it forces the log, and now we push the CIL
> > without sufficient reservation to write out the log headers and the
> > items we just formatted....
> > 
> 
> Hmmm... that seems like an odd scenario because I'd expect the space
> usage delta to reflect what might or might not have already been added
> to the CIL context, not necessarily the AIL. IOW, shouldn't a negative
> delta only occur for items being relogged while still CIL resident
> (regardless of AIL residency)?
> 
> From a code standpoint, the way a particular log item delta comes out
> negative is from having a shadow lv size smaller than the ->li_lv size.
> Thus, xlog_cil_insert_format_items() subtracts the currently formatted
> lv size from the delta, formats the current state of the item, and
> xfs_cil_prepare_item() adds the new (presumably smaller) size to the
> delta. We reuse ->li_lv in this scenario so both it and the shadow
> buffer remain, but a CIL push pulls ->li_lv from all log items and
> chains them to the CIL context for writing, so I don't see how we could
> have an item return a negative delta on an empty CIL. Hm?
> 
> (I was also wondering whether repeated smaller relogs of an item could
> be a vector for this to go wrong, but it looks like
> xlog_cil_insert_format_items() always uses the formatted size of the
> current buffer...).
> 
> Brian
> 
> > So, yeah, shrinking transaction space usage definitely violates some
> > of the assumptions the code makes about how relogging works. It's
> > entirely possible the assumptions I've made are not entirely correct
> > in some corner cases - those particular cases are what we need to
> > ferret out here, and then decide if they are correct or not and deal
> > with it from there...
> > 
> > Cheers,
> > 
> > Dave.
> > -- 
> > Dave Chinner
> > david@fromorbit.com
> > 
>