Re: [PATCH 1/8] xfs: push the AIL in xlog_grant_head_wake

Dave Chinner
2019-09-05T22:02:33Z

On Thu, Sep 05, 2019 at 08:18:28AM -0700, Darrick J. Wong wrote:
> On Thu, Sep 05, 2019 at 06:47:10PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > In the situation where the log is full and the CIL has not recently
> > flushed, the AIL push threshold is throttled back to the where the
> > last write of the head of the log was completed. This is stored in
> > log->l_last_sync_lsn. Hence if the CIL holds > 25% of the log space
> > pinned by flushes and/or aggregation in progress, we can get the
> > situation where the head of the log lags a long way behind the
> > reservation grant head.
> >
> > When this happens, the AIL push target is trimmed back from where
> > the reservation grant head wants to push the log tail to, back to
> > where the head of the log currently is. This means the push target
> > doesn't reach far enough into the log to actually move the tail
> > before the transaction reservation goes to sleep.
> >
> > When the CIL push completes, it moves the log head forward such that
> > the AIL push target can now be moved, but that has no mechanism for
> > puhsing the log tail. Further, if the next tail movement of the log
> > is not large enough wake the waiter (i.e. still not enough space for
> > it to have a reservation granted), we don't wake anything up, and
> > hence we do not update the AIL push target to take into account the
> > head of the log moving and allowing the push target to be moved
> > forwards.
> >
> > To avoid this particular condition, if we fail to wake the first
> > waiter on the grant head because we don't have enough space,
> > push on the AIL again. This will pick up any movement of the log
> > head and allow the push target to move forward due to completion of
> > CIL pushing.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> > fs/xfs/xfs_log.c | 23 ++++++++++++++++++++++-
> > 1 file changed, 22 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> > index b159a9e9fef0..5e21450fb8f5 100644
> > --- a/fs/xfs/xfs_log.c
> > +++ b/fs/xfs/xfs_log.c
> > @@ -214,15 +214,36 @@ xlog_grant_head_wake(
> > {
> > struct xlog_ticket *tic;
> > int need_bytes;
> > + bool woken_task = false;
> >
> > list_for_each_entry(tic, &head->waiters, t_queue) {
> > + /*
> > + * The is a chance that the size of the CIL checkpoints in
> > + * progress result at the last AIL push resulted in the log head
> > + * (l_last_sync_lsn) not reflecting where the log head now is.
>
> That's a bit difficult to understand...

Yup I failed to edit it properly and left an extra "result" in the
sentence...

> "There is a chance that the size of the CIL checkpoints in progress at
> the last AIL push results in the last committed log head (l_last_sync_lsn)
> not reflecting where the log head is now." ?
>
> (Did I get that right?)

*nod*.

> > + * Hence when we are woken here, it may be the head of the log
> > + * that has moved rather than the tail. In that case, the tail
> > + * didn't move and there won't be space available until the AIL
> > + * push target is updated and the tail pushed again. If the AIL
> > + * is already pushed to it's target, we will hang here until
>
> Nit: "its", not "it is".
>
> Other than that I think I can tell what this is doing now. :)

In reading this again, it is all a bit clunky. I've revised it a bit
more to more concisely describe the situation:

/*
* The is a chance that the size of the CIL checkpoints in
* progress at the last AIL push target calculation resulted in
* limiting the target to the log head (l_last_sync_lsn) at the
* time. This may not reflect where the log head is now as the
* CIL checkpoints may have completed.
*
* Hence when we are woken here, it may be that the head of the
* log that has moved rather than the tail. As the tail didn't
* move, there still won't be space available for the
* reservation we require. However, if the AIL has already
* pushed to the target defined by the old log head location, we
* will hang here waiting for something else to update the AIL
* push target.
*
* Therefore, if there isn't space to wake the first waiter on
* the grant head, we need to push the AIL again to ensure the
* target reflects both the current log tail and log head
* position before we wait for the tail to move again.
*/

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com