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@xxxxxxxxxxxxx> > > > > > --- > > > > > 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@xxxxxxxxxxxxx > > >
