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

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 03, 2021 at 12:20:19PM -0500, Brian Foster wrote:
> On Wed, Mar 03, 2021 at 12:23:04PM +1100, Dave Chinner wrote:
> > On Mon, Mar 01, 2021 at 08:32:18AM -0500, Brian Foster wrote:
> > > On Mon, Mar 01, 2021 at 03:54:22PM +1100, Dave Chinner wrote:
> > > > On Sat, Feb 27, 2021 at 11:25:06AM -0500, Brian Foster wrote:
> > > > > On Fri, Feb 26, 2021 at 09:03:05AM +1100, Dave Chinner wrote:
> > > > > > This is really nasty behaviour, and it's only recently that I've got
> > > > > > a handle on it. I found it because my original "async CIL push" code
> > > > > > resulted in long stalls every time the log is filled and the tail is
> > > > > > pinned by a buffer that is being relogged in this manner....
> > > > > > 
> > > > > > I'm not sure how to fix this yet - the AIL needs to block the front
> > > > > > end relogging to allow the buffer to be unpinned. Essentially, we
> > > > > > need to hold the pinned items locked across a CIL push to guarantee
> > > > > > they are unpinned, but that's the complete opposite of what the AIL
> > > > > > currently does to prevent the front end from seeing long tail lock
> > > > > > latencies when modifying stuff....
> > > > > 
> > > > > When this stall problem manifests, I'm assuming it's exacerbated by
> > > > > delayed logging and the commit record behavior I described above. If
> > > > > that's the case, could the AIL communicate writeback pressure through
> > > > > affected log items such that checkpoints in which they are resident are
> > > > > flushed out completely/immediately when the checkpoints occur? I suppose
> > > > > that would require a log item flag or some such, which does raise a
> > > > > concern of unnecessarily tagging many items (it's not clear to me how
> > > > > likely that really is), but I'm curious if that would be an effective
> > > > > POC at least..
> > > > 
> > > > I don't think we need to do anything like that. All we need to do to
> > > > ensure that the AIL can flush a pinned buffer is to lock it, kick
> > > > the log and wait for the pin count to go to zero. Then we can write
> > > > it just fine, blocking only the front end transactions that need
> > > > that buffer lock.  Same goes for inodes, though xfs_iunpin_wait()
> > > > already does this....
> > > > 
> > > 
> > > Yeah, but why would we want to block xfsaild on a single item like that?
> > 
> > Who said anything about blocking the AIL on a single item? :)
> > 
> > > Wouldn't holding the item locked like that just create a new stall point
> > > within xfsaild? Maybe I'm missing something, but what you describe here
> > > basically just sounds like a "lock the item and run a sync log force"
> > > pattern.
> > 
> > What I was thinking is that if the item is pinned and at the
> > tail of the log, then we leave it locked when we flush it rather
> > than unlocking it and relocking it when the delwri submit code gets
> > to it. If it gets unpinned before the delwri code gets to it, all
> > good. If not, the delwri code being unable to flush it will feed
> > back up into the AIL to trigger a log force, which will unpin it
> > in the near future and it will be written on the next AIL delwri
> > submit cycle.
> > 
> 
> I'm not sure what you mean by leaving the item locked when we flush it
> if it is pinned, since we don't flush pinned items.

That's exactly what I'm talking about changing.

-Dave.
-- 
Dave Chinner
david@fromorbit.com