Re: [PATCH RFC 3/4] xfs: crude chunk allocation retry mechanism

From: Brian Foster <bfoster@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH RFC 3/4] xfs: crude chunk allocation retry mechanism
Date: Fri, 18 Feb 2022 09:21:40 -0500	[thread overview]
Message-ID: <Yg+rdFRpvra8U25D@bfoster> (raw)
In-Reply-To: <20220217232033.GD59715@dread.disaster.area>

On Fri, Feb 18, 2022 at 10:20:33AM +1100, Dave Chinner wrote:
> On Thu, Feb 17, 2022 at 12:25:17PM -0500, Brian Foster wrote:
> > The free inode btree currently tracks all inode chunk records with
> > at least one free inode. This simplifies the chunk and allocation
> > selection algorithms as free inode availability can be guaranteed
> > after a few simple checks. This is no longer the case with busy
> > inode avoidance, however, because busy inode state is tracked in the
> > radix tree independent from physical allocation status.
> > 
> > A busy inode avoidance algorithm relies on the ability to fall back
> > to an inode chunk allocation one way or another in the event that
> > all current free inodes are busy. Hack in a crude allocation
> > fallback mechanism for experimental purposes. If the inode selection
> > algorithm is unable to locate a usable inode, allow it to return
> > -EAGAIN to perform another physical chunk allocation in the AG and
> > retry the inode allocation.
> > 
> > The current prototype can perform this allocation and retry sequence
> > repeatedly because a newly allocated chunk may still be covered by
> > busy in-core inodes in the radix tree (if it were recently freed,
> > for example). This is inefficient and temporary. It will be properly
> > mitigated by background chunk removal. This defers freeing of inode
> > chunk blocks from the free of the last used inode in the chunk to a
> > background task that only frees chunks once completely idle, thereby
> > providing a guarantee that a new chunk allocation always adds
> > non-busy inodes to the AG.
> 
> I think you can get rid of this simply by checking the radix tree
> tags for busy inodes at the location of the new inode chunk before
> we do the cluster allocation. If there are busy inodes in the range
> of the chunk (pure gang tag lookup, don't need to dereference any of
> the inodes), just skip to the next chunk offset and try that. Hence
> we only ever end up allocating a chunk that we know there are no
> busy inodes in and this retry mechanism is unnecessary.
> 

The retry mechanism exists in this series due to the factoring of the
inode allocation code moreso than whether the fallback is guaranteed to
provide a fully non-busy chunk or not. As the prototype is written, the
inode scan still needs to fall back at least once even with such a
guarantee (see my reply on the previous patch around cleaning up that
particular wart).

With regard to checking busy inode state, that is pretty much what I was
referring to by filtering or hinting the block allocation when we
discussed this on IRC. I'm explicitly trying to avoid that because for
one it unnecessarily spreads concern about busy inodes across layers. On
top of that, it assumes that there will always be some usable physical
block range available without busy inodes, which is not the case. That
means we now need to consider the fact that chunk allocation might fail
for reasons other than -ENOSPC and factor that into the inode allocation
algorithm. IOW, ISTM this just unnecessarily complicates things for
minimal benefit.

The point of background freeing inode chunks was that it makes this
problem go away because then we ensure that inode chunks aren't freed
until all associated busy inodes are cleared, and so we preserve the
historical behavior that an inode chunk allocation guarantees immediate
ability to allocate an inode. I thought we agreed in the previous
discussion that this was the right approach since it seemed to be in the
long term direction for XFS anyways.. hm?

Brian

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