Re: [PATCH 8/8] shmem,percpu_counter: add _limited_add(fbc, limit, amount)

[Hugh Dickins <hughd@google.com>]

On Mon, 9 Oct 2023, Dave Chinner wrote:
> On Thu, Oct 05, 2023 at 10:35:33PM -0700, Hugh Dickins wrote:
> > On Thu, 5 Oct 2023, Dave Chinner wrote:
> > > 
> > > Hmmmm. IIUC, this only works for addition that approaches the limit
> > > from below?
> > 
> > That's certainly how I was thinking about it, and what I need for tmpfs.
> > Precisely what its limitations (haha) are, I'll have to take care to
> > spell out.
> > 
> > (IIRC - it's a while since I wrote it - it can be used for subtraction,
> > but goes the very slow way when it could go the fast way - uncompared
> > percpu_counter_sub() much better for that.  You might be proposing that
> > a tweak could adjust it to going the fast way when coming down from the
> > "limit", but going the slow way as it approaches 0 - that would be neat,
> > but I've not yet looked into whether it's feasily done.)

Easily done once I'd looked at it from the right angle.

> > 
> > > 
> > > So if we are approaching the limit from above (i.e. add of a
> > > negative amount, limit is zero) then this code doesn't work the same
> > > as the open-coded compare+add operation would?
> > 
> > To it and to me, a limit of 0 means nothing positive can be added
> > (and it immediately returns false for that case); and adding anything
> > negative would be an error since the positive would not have been allowed.
> > 
> > Would a negative limit have any use?

There was no reason to exclude it, once I was thinking clearly
about the comparisons.

> 
> I don't have any use for it, but the XFS case is decrementing free
> space to determine if ENOSPC has been hit. It's the opposite
> implemention to shmem, which increments used space to determine if
> ENOSPC is hit.

Right.

> 
> > It's definitely not allowing all the possibilities that you could arrange
> > with a separate compare and add; whether it's ruling out some useful
> > possibilities to which it can easily be generalized, I'm not sure.
> > 
> > Well worth a look - but it'll be easier for me to break it than get
> > it right, so I might just stick to adding some comments.
> > 
> > I might find that actually I prefer your way round: getting slower
> > as approaching 0, without any need for specifying a limit??  That the
> > tmpfs case pushed it in this direction, when it's better reversed?  Or
> > that might be an embarrassing delusion which I'll regret having mentioned.
> 
> I think there's cases for both approaching and upper limit from
> before and a lower limit from above. Both are the same "compare and
> add" algorithm, just with minor logic differences...

Good, thanks, you've saved me: I was getting a bit fundamentalist there,
thinking to offer one simplest primitive from which anything could be
built.  But when it came down to it, I had no enthusiam for rewriting
tmpfs's used_blocks as free_blocks, just to avoid that limit argument.

> 
> > > Hence I think this looks like a "add if result is less than"
> > > operation, which is distinct from then "add if result is greater
> > > than" operation that we use this same pattern for in XFS and ext4.
> > > Perhaps a better name is in order?
> > 
> > The name still seems good to me, but a comment above it on its
> > assumptions/limitations well worth adding.
> > 
> > I didn't find a percpu_counter_compare() in ext4, and haven't got
> 
> Go search for EXT4_FREECLUSTERS_WATERMARK....

Ah, not a percpu_counter_compare() user, but doing its own thing.

> 
> > far yet with understanding the XFS ones: tomorrow...
> 
> XFS detects being near ENOSPC to change the batch update size so
> taht when near ENOSPC the percpu counter always aggregates to the
> global sum on every modification. i.e. it becomes more accurate (but
> slower) near the ENOSPC threshold. Then if the result of the
> subtraction ends up being less than zero, it takes a lock (i.e. goes
> even slower!), undoes the subtraction that took it below zero, and
> determines if it can dip into the reserve pool or ENOSPC should be
> reported.
> 
> Some of that could be optimised, but we need that external "lock and
> undo" mechanism to manage the reserve pool space atomically at
> ENOSPC...

Thanks for going above and beyond with the description; but I'll be
honest and admit that I only looked quickly, and did not reach any
conclusion as to whether such usage could or should be converted
to percpu_counter_limited_add() - which would never take any XFS
locks, of course, so might just end up doubling the slow work.

But absolutely I agree with you, and thank you for pointing out,
how stupidly useless percpu_counter_limited_add() was for decrementing -
it was nothing more than a slow way of doing percpu_counter_sub().

I'm about to send in a 9/8, extending it to be more useful: thanks.

Hugh