Re: [PATCH 2/3 v2] xfs: AIL needs asynchronous CIL forcing

[Brian Foster <bfoster@redhat.com>]

On Thu, Mar 04, 2021 at 12:59:33PM +1100, Dave Chinner wrote:
> On Wed, Mar 03, 2021 at 12:32:39PM -0500, Brian Foster wrote:
> > On Wed, Mar 03, 2021 at 11:57:52AM +1100, Dave Chinner wrote:
> > > On Tue, Mar 02, 2021 at 04:44:12PM -0500, Brian Foster wrote:
> > > > On Thu, Feb 25, 2021 at 10:26:00AM +1100, Dave Chinner wrote:
> > > > >  	 * xlog_cil_push() handles racing pushes for the same sequence,
> > > > >  	 * so no need to deal with it here.
> > > > >  	 */
> > > > >  restart:
> > > > > -	xlog_cil_push_now(log, sequence);
> > > > > +	xlog_cil_push_now(log, sequence, flags & XFS_LOG_SYNC);
> > > > > +	if (!(flags & XFS_LOG_SYNC))
> > > > > +		return commit_lsn;
> > > > 
> > > > Hm, so now we have sync and async log force and sync and async CIL push.
> > > > A log force always requires a sync CIL push and commit record submit;
> > > > the difference is simply whether or not we wait on commit record I/O
> > > > completion. The sync CIL push drains the CIL of log items but does not
> > > > switch out the commit record iclog, while the async CIL push executes in
> > > > the workqueue context so the drain is async, but it does switch out the
> > > > commit record iclog before it completes. IOW, the async CIL push is
> > > > basically a "more async" async log force.
> > > 
> > > Yes.
> > > 
> > > > I can see the need for the behavior of the async CIL push here, but that
> > > > leaves a mess of interfaces and behavior matrix. Is there any reason we
> > > > couldn't just make async log forces unconditionally behave equivalent to
> > > > the async CIL push as defined by this patch? There's only a handful of
> > > > existing users and I don't see any obvious reason why they might care
> > > > whether the underlying CIL push is synchronous or not...
> > > 
> > > I'm not touching the rest of the log force code in this series - it
> > > is out of scope of this bug fix and the rest of the series that it
> > > is part of.
> > > 
> > 
> > But you already have altered the log force code by changing
> > xlog_cil_force_seq(), which implicitly changes how xfs_log_force_seq()
> > behaves.
> 
> The behaviour of the function when called from xfs_log_force*()
> should be unchanged. Can you be specific about exactly what
> behaviour did I change for non-synchronous xfs_log_force*() callers
> so I can fix it? I have not intended to change it at all, so
> whatever you are refering is an issue I need to fix...
> 

xfs_log_force_seq() passes flags from the caller to xlog_cil_force_seq()
(whereas this patch presumably wants it to pass XFS_LOG_SYNC
unconditionally). IOW, xfs_log_force(mp, 0) behavior is different from
xfs_log_force_seq(mp, seq, 0, ...).

> > So not only does this patch expose subsystem internals to
> > external layers and create more log forcing interfaces/behaviors to
> 
> Sorry, I don't follow. What "subsystem internals" are being exposed
> and what external layer are they being exposed to?
> 
> > > Cleaning up the mess that is the xfs_log_* and xlog_* interfaces and
> > > changing things like log force behaviour and implementation is for
> > > a future series.
> > > 
> > 
> > TBH I think this patch is kind of a mess on its own. I think the
> > mechanism it wants to provide is sane, but I've not even got to the
> > point of reviewing _that_ yet because of the seeming dismissal of higher
> > level feedback. I'd rather not go around in circles on this so I'll just
> > offer my summarized feedback to this patch...
> 
> I'm not dismissing review nor am I saying the API cannot or should
> not be improved. I'm simply telling you that I think the additional
> changes you are proposing are outside the scope of the problem I am
> addressing here. I already plan to rework the log force API (and
> others) but doing so it not something that this patchset needs to
> address, or indeed should address.
> 

I'm not proposing additional changes nor to rework the log force API.
I'm pointing out that I find this implementation to be extremely and
unnecessarily confusing. To improve it, I'm suggesting to either coopt
the existing async log force API...

> There are already enough subtle changes being made to core code and
> algorithms that mixing them with unrelated high level external
> behavioural changes that it greatly increases the risk of unexpected
> regressions in the patchset. The log force are paths are used in
> data integrity paths, so I want to limit the scope of behavioural
> change to just the AIL where the log force has no data integrity
> requirement associcated with it.
> 

... or if we really want a special async log force just for xfsaild (why
is still not clear to me), then tie it to an XFS_LOG_REALLY_ASYNC flag
or some such, pass that to the existing log force call, and document the
purpose/behavior of the new mode in detail. That at least won't require
a developer to wonder what !(flags & XFS_LOG_SYNC) happens to mean
depending on the particular log force function.

Brian

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
>