Re: [PATCH 13/24] xfs: synchronous AIL pushing

Brian Foster
2019-08-06T12:29:53Z

On Tue, Aug 06, 2019 at 09:21:32AM +1000, Dave Chinner wrote:
> On Mon, Aug 05, 2019 at 01:51:53PM -0400, Brian Foster wrote:
> > On Thu, Aug 01, 2019 at 12:17:41PM +1000, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > >
> > > Provide an interface to push the AIL to a target LSN and wait for
> > > the tail of the log to move past that LSN. This is used to wait for
> > > all items older than a specific LSN to either be cleaned (written
> > > back) or relogged to a higher LSN in the AIL. The primary use for
> > > this is to allow IO free inode reclaim throttling.
> > >
> > > Factor the common AIL deletion code that does all the wakeups into a
> > > helper so we only have one copy of this somewhat tricky code to
> > > interface with all the wakeups necessary when the LSN of the log
> > > tail changes.
> > >
> > > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > > ---
> > > fs/xfs/xfs_inode_item.c | 12 +------
> > > fs/xfs/xfs_trans_ail.c | 69 +++++++++++++++++++++++++++++++++--------
> > > fs/xfs/xfs_trans_priv.h | 6 +++-
> > > 3 files changed, 62 insertions(+), 25 deletions(-)
> > >
> > ...
> > > diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> > > index 6ccfd75d3c24..9e3102179221 100644
> > > --- a/fs/xfs/xfs_trans_ail.c
> > > +++ b/fs/xfs/xfs_trans_ail.c
> > > @@ -654,6 +654,37 @@ xfs_ail_push_all(
> > > xfs_ail_push(ailp, threshold_lsn);
> > > }
> > >
> > > +/*
> > > + * Push the AIL to a specific lsn and wait for it to complete.
> > > + */
> > > +void
> > > +xfs_ail_push_sync(
> > > + struct xfs_ail *ailp,
> > > + xfs_lsn_t threshold_lsn)
> > > +{
> > > + struct xfs_log_item *lip;
> > > + DEFINE_WAIT(wait);
> > > +
> > > + spin_lock(&ailp->ail_lock);
> > > + while ((lip = xfs_ail_min(ailp)) != NULL) {
> > > + prepare_to_wait(&ailp->ail_push, &wait, TASK_UNINTERRUPTIBLE);
> > > + if (XFS_FORCED_SHUTDOWN(ailp->ail_mount) ||
> > > + XFS_LSN_CMP(threshold_lsn, lip->li_lsn) <= 0)
> > > + break;
> > > + /* XXX: cmpxchg? */
> > > + while (XFS_LSN_CMP(threshold_lsn, ailp->ail_target) > 0)
> > > + xfs_trans_ail_copy_lsn(ailp, &ailp->ail_target, &threshold_lsn);
> >
> > Why the need to repeatedly copy the ail_target like this? If the push
>
> It's a hack because the other updates are done unlocked and this
> doesn't contain the memroy barriers needed to make it correct
> and race free.
>
> Hence the comment "XXX: cmpxchg" to ensure that:
>
> a) we only ever move the target forwards;
> b) we resolve update races in an obvious, simple manner; and
> c) we can get rid of the possibly incorrect memory
> barriers around this (unlocked) update.
>
> RFC. WIP. :)
>

Ack..

> > target only ever moves forward, we should only need to do this once at
> > the start of the function. In fact I'm kind of wondering why we can't
> > just call xfs_ail_push(). If we check the tail item after grabbing the
> > spin lock, we should be able to avoid any races with the waker, no?
>
> I didn't use xfs_ail_push() because of having to prepare to wait
> between determining if the AIL is empty and checking if we need
> to update the target.
>
> I also didn't want to affect the existing xfs_ail_push() as I was
> modifying the xfs_ail_push_sync() code to do what was needed.
> Eventually they can probably come back together, but for now I'm not
> 100% sure that the code is correct and race free.
>

Ok, just chalk this up to general feedback around seeing if we can
improve the factoring between the several AIL pushing functions we now
have once this mechanism is working/correct.

> > > +void
> > > +xfs_ail_delete_finish(
> > > + struct xfs_ail *ailp,
> > > + bool do_tail_update) __releases(ailp->ail_lock)
> > > +{
> > > + struct xfs_mount *mp = ailp->ail_mount;
> > > +
> > > + if (!do_tail_update) {
> > > + spin_unlock(&ailp->ail_lock);
> > > + return;
> > > + }
> > > +
> >
> > Hmm.. so while what we really care about here are tail updates, this
> > logic is currently driven by removing the min ail log item. That seems
> > like a lot of potential churn if we're waking the pusher on every object
> > written back covered by a single log record / checkpoint. Perhaps we
> > should implement a bit more coarse wakeup logic such as only when the
> > tail lsn actually changes, for example?
>
> You mean the next patch?
>

Yep, hadn't got to that point yet. FWIW, I think the next patch should
probably come before this one so there isn't a transient state with
whatever behavior results from this patch by itself.

Brian

> > FWIW, it also doesn't look like you've handled the case of relogged
> > items moving the tail forward anywhere that I can see, so we might be
> > missing some wakeups here. See xfs_trans_ail_update_bulk() for
> > additional AIL manipulation.
>
> Good catch. That might be the race the next patch exposes :)
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com