On Fri, 31 Jul 2015, Jan Kara wrote: > Date: Fri, 31 Jul 2015 14:39:17 +0200 > From: Jan Kara <jack@xxxxxxx> > To: Lukáš Czerner <lczerner@xxxxxxxxxx> > Cc: Jan Kara <jack@xxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx > Subject: Re: [PATCH] jbd2: Limit number of reserved credits > > On Fri 31-07-15 13:07:59, Lukáš Czerner wrote: > > On Fri, 31 Jul 2015, Lukáš Czerner wrote: > > > > > Date: Fri, 31 Jul 2015 12:56:55 +0200 (CEST) > > > From: Lukáš Czerner <lczerner@xxxxxxxxxx> > > > To: Jan Kara <jack@xxxxxxx> > > > Cc: linux-ext4@xxxxxxxxxxxxxxx > > > Subject: Re: [PATCH] jbd2: Limit number of reserved credits > > > > > > On Fri, 31 Jul 2015, Jan Kara wrote: > > > > > > > Date: Fri, 31 Jul 2015 12:46:04 +0200 > > > > From: Jan Kara <jack@xxxxxxx> > > > > To: Lukáš Czerner <lczerner@xxxxxxxxxx> > > > > Cc: Jan Kara <jack@xxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx > > > > Subject: Re: [PATCH] jbd2: Limit number of reserved credits > > > > > > > > On Fri 31-07-15 12:22:43, Lukáš Czerner wrote: > > > > > On Fri, 31 Jul 2015, Jan Kara wrote: > > > > > > > > > > > Date: Fri, 31 Jul 2015 11:46:39 +0200 > > > > > > From: Jan Kara <jack@xxxxxxx> > > > > > > To: Lukas Czerner <lczerner@xxxxxxxxxx> > > > > > > Cc: linux-ext4@xxxxxxxxxxxxxxx, jack@xxxxxxx > > > > > > Subject: Re: [PATCH] jbd2: Limit number of reserved credits > > > > > > > > > > > > Hello, > > > > > > > > > > > > On Fri 31-07-15 10:04:23, Lukas Czerner wrote: > > > > > > > Currently there is no limitation on number of reserved credits we can > > > > > > > ask for. If we ask for more reserved credits than 1/2 of maximum > > > > > > > transaction size, or if total number of credits exceeds the maximum > > > > > > > transaction size per operation (which is currently only possible with > > > > > > > the former) we will spin forever in start_this_handle(). > > > > > > > > > > > > > > Fix this by adding this limitation at the start of start_this_handle(). > > > > > > > > > > > > > > This patch also removes the credit limitation 1/2 of maximum transaction > > > > > > > size, since we really only want to limit the number of reserved credits. > > > > > > > There is not much point to limit the credits if there is still space in > > > > > > > the journal. > > > > > > > > > > > > > > This accidentally also fixes the online resize, where due to the > > > > > > > limitation of the journal credits we're unable to grow file systems with > > > > > > > 1k block size and size between 16M and 32M. It has been partially fixed > > > > > > > by 2c869b262a10ca99cb866d04087d75311587a30c, but not entirely. > > > > > > > > > > > > > > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx> > > > > > > > --- > > > > > > > > > > > > > > Honzo I think that this should be enough to remove the limitation of 1/2 of > > > > > > > maximum transaction size for regular credits, but I might be missing > > > > > > > something, please let me know. Also do you have any specific test case to > > > > > > > exercise transaction reservation support - I've only ran xfstests. > > > > > > > > > > > > > > fs/jbd2/transaction.c | 22 +++++++++++++--------- > > > > > > > 1 file changed, 13 insertions(+), 9 deletions(-) > > > > > > > > > > > > > > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c > > > > > > > index f3d0617..491a328 100644 > > > > > > > --- a/fs/jbd2/transaction.c > > > > > > > +++ b/fs/jbd2/transaction.c > > > > > > > @@ -262,20 +262,24 @@ static int start_this_handle(journal_t *journal, handle_t *handle, > > > > > > > int rsv_blocks = 0; > > > > > > > unsigned long ts = jiffies; > > > > > > > > > > > > > > + if (handle->h_rsv_handle) > > > > > > > + rsv_blocks = handle->h_rsv_handle->h_buffer_credits; > > > > > > > + > > > > > > > /* > > > > > > > - * 1/2 of transaction can be reserved so we can practically handle > > > > > > > - * only 1/2 of maximum transaction size per operation > > > > > > > + * Limit the number of reserved credits to 1/2 of maximum transaction > > > > > > > + * size and limit the number of total credits to not exceed maximum > > > > > > > + * transaction size per operation. > > > > > > > */ > > > > > > > - if (WARN_ON(blocks > journal->j_max_transaction_buffers / 2)) { > > > > > > > - printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n", > > > > > > > - current->comm, blocks, > > > > > > > - journal->j_max_transaction_buffers / 2); > > > > > > > + if ((rsv_blocks > journal->j_max_transaction_buffers / 2) || > > > > > > > + (rsv_blocks + blocks > journal->j_max_transaction_buffers)) { > > > > > > > + printk(KERN_ERR "JBD2: %s wants too many credits " > > > > > > > + "credits:%d rsv_credits:%d max:%d\n", > > > > > > > + current->comm, blocks, rsv_blocks, > > > > > > > + journal->j_max_transaction_buffers); > > > > > > > + WARN_ON(1); > > > > > > > return -ENOSPC; > > > > > > > } > > > > > > > > > > > > Well, the trouble with this is the following: The currently running > > > > > > transaction has X reserved credits and Y normal credits. We know X+Y <= > > > > > > journal->j_max_transaction_buffers. Now you request additional A reserved > > > > > > and B normal credits. Suppose we cannot fit in the current transaction - > > > > > > i.e., X+Y+A+B > journal->j_max_transaction_buffers. The only thing we can do > > > > > > is to push running transaction to commit and start a new one. However, the > > > > > > new transaction will also have X reserved credits - you inherit reserved > > > > > > credits from the previous transaction until they are converted to normal > > > > > > credits. So if X+A+B is still > journal->j_max_transaction_buffers, you > > > > > > still cannot start current handle and you'd have to wait until someone > > > > > > converts his reserved credits. > > > > > > > > > > Ok I understand, but isn't this true either way ? If anything the > > > > > limit might make it worse in that case because if > > > > > > > > > > X+A+B is still > journal->j_max_transaction_buffers > > > > > > > > > > in the new case without the limit then it's definitely true for the > > > > > case with the limit as well. The number of reserved credits is > > > > > limited in both cases so it's not really a factor, is it ? > > > > > > > > > > Yes in the limitless case it might happen that we have so much > > > > > normal credits that we can't fit in the reserved credits so we have > > > > > to commit and start a new one, but that's true in both cases only > > > > > with the limit it will happen sooner and possible more often because > > > > > we just have less space to work with. > > > > > > > > > > Sorry if I am asking dumb questions, but I am trying to understand > > > > > how is this supposed to work. > > > > > > > > > > And above all that limitation we're talking about is a hard limit > > > > > which you're not supposed to hit ever. Only if something is really > > > > > wrong and is asking for a handle with way too much credits...that's > > > > > not what can normally happen. So what's the problem again ? > > > > > > > > Thanks for correcting me! I was conflating two different conditions in the > > > > transaction handling code. So with the change you propose, it would be only > > > > possible that starting of large handles would keep pushing transactions to > > > > commit because it couldn't fit the handle into the running transaction > > > > because of reserved credits. So if we wanted to relieve the condition as > > > > you suggest, we'd also need to modify the logic in > > > > add_transaction_credits() to wait on j_wait_reserved in case number of > > > > reserved credits of current trans + number of credits requested for the handle > > > > is too big. But that looks doable... > > > > > > Ah, right. Thanks, I'll resend the patch. > > > > One more question tough. Since we already check for available blocks > > in the transaction in add_transaction_credits() with: > > > > needed = atomic_add_return(total, &t->t_outstanding_credits); > > if (needed > journal->j_max_transaction_buffers) { > > ... > > > > then we know that we have enough space, so we just need to make sure > > The trouble happens when needed > journal->j_max_transaction_buffers. Then > wait_transaction_locked(journal) doesn't necessarily guarantee forward > progress if "total > journal->j_max_transaction_buffers / 2" since we can > have journal->j_max_transaction_buffers / 2 credits reserved and thus we > would loop forcing transaction commits. Even now things are actually > slightly buggy because we only verify number of normal credits is <= > journal->j_max_transaction_buffers / 2, not 'total' so there's some > potential for busy loop even now. > > Waits in that condition would need to be like: > > atomic_sub(total, &t->t_outstanding_credits); > /* > * Is the number of reserved credits in the current transaction too > * big to fit this handle? Wait until reserved credits are freed. > */ > if (atomic_read(&journal->j_reserved_credits) + total > > journal->j_max_transaction_buffers) { > read_unlock(&journal->j_state_lock); > wait_event(journal->j_wait_reserved, > atomic_read(&journal->j_reserved_credits) + total <= > journal->j_max_transaction_buffers); > return 1; > } > /* > * OK, if we push current transaction to commit, we should have > * enough space for our handle. > */ > wait_transaction_locked(journal); > return 1; Makes sense, Thanks! -Lukas > > Honza > > > that number of reserved credits is limited to 1/2 of > > j_max_transaction_buffers right ? So nothing more is needed except of > > maybe making sure that this is still true while we wait in case the > > reserved credits exceed 1/2 of the transaction. Something like this > > in the "if (needed > journal->j_max_transaction_buffers / 2)" > > condition should be enough ? > > > > wait_event(journal->j_wait_reserved, > > atomic_read(&journal->j_reserved_credits) + total > > <= journal->j_max_transaction_buffers); > > > > though I am not entirely sure this is necessary. > > > > Thanks! > > -Lukas > > > > > > > > -Lukas > > > > > > > > > > > Honza > > > > > > > > > > > > > > Thanks! > > > > > -Lukas > > > > > > > > > > > > > > > > > > > > > > However these waits will create journal stalls causing possible performance > > > > > > issues and also introduce a lock dependency - suddently you are not allowed > > > > > > to acquire locks ranking above transaction start before starting a reserved > > > > > > handle (as these locks can be held by processes being stuck waiting for > > > > > > reserved credits to convert). > > > > > > > > > > > > So overall halving the maximum allowed credits seemed like the least > > > > > > painful solution to the problem. > > > > > > > > > > > > Honza > > > > > > > > > > > > > > - if (handle->h_rsv_handle) > > > > > > > - rsv_blocks = handle->h_rsv_handle->h_buffer_credits; > > > > > > > - > > > > > > > alloc_transaction: > > > > > > > if (!journal->j_running_transaction) { > > > > > > > /* > > > > > > > -- > > > > > > > 1.8.3.1 > > > > > > > > > > > > > > > > > > >