On Wed, Feb 17, 2021 at 08:23:39AM -0500, Brian Foster wrote:

The blocks used for allocation btrees (bnobt and countbt) are
technically considered free space. This is because as free space is
used, allocbt blocks are removed and naturally become available for
traditional allocation. However, this means that a significant
portion of free space may consist of in-use btree blocks if free
space is severely fragmented.

On large filesystems with large perag reservations, this can lead to
a rare but nasty condition where a significant amount of physical
free space is available, but the majority of actual usable blocks
consist of in-use allocbt blocks. We have a record of a (~12TB, 32
AG) filesystem with multiple AGs in a state with ~2.5GB or so free
blocks tracked across ~300 total allocbt blocks, but effectively at
100% full because the the free space is entirely consumed by
refcountbt perag reservation.

Such a large perag reservation is by design on large filesystems.
The problem is that because the free space is so fragmented, this AG
contributes the 300 or so allocbt blocks to the global counters as
free space. If this pattern repeats across enough AGs, the
filesystem lands in a state where global block reservation can
outrun physical block availability. For example, a streaming
buffered write on the affected filesystem continues to allow delayed
allocation beyond the point where writeback starts to fail due to
physical block allocation failures. The expected behavior is for the
delalloc block reservation to fail gracefully with -ENOSPC before
physical block allocation failure is a possibility.

*nod*

To address this problem, introduce a percpu counter to track the sum
of the allocbt block counters already tracked in the AGF. Use the
new counter to set these blocks aside at reservation time and thus
ensure they cannot be allocated until truly available. Since this is
only necessary when large reflink perag reservations are in place
and the counter requires a read of each AGF to fully populate, only
enforce on reflink enabled filesystems. This allows initialization
of the counter at ->pagf_init time because the refcountbt perag
reservation init code reads each AGF at mount time.

Ok, so the mechanism sounds ok, but a per-cpu counter seems like
premature optimisation. How often are we really updating btree block
counts? An atomic counter is good for at least a million updates a
second across a 2 socket 32p machine, and I highly doubt we're
incrementing/decrementing btree block counts that often on such a
machine. 

While per-cpu counters have a fast write side, they come with
additional algorithmic complexity. Hence if the update rate of the
counter is not fast enough to need per-cpu counters, we should avoid
them. just because other free space counters use per-cpu counters,
it doesn't mean everything in that path needs to use them...

Note that the counter uses a small percpu batch size to allow the
allocation paths to keep the primary count accurate enough that the
reservation path doesn't ever need to lock and sum the counter.
Absolute accuracy is not required here, just that the counter
reflects the majority of unavailable blocks so the reservation path
fails first.

And this makes the per-cpu counter scale almost no better than an
simple atomic counter, because a spinlock requires two atomic
operations (lock and unlock). Hence a batch count of 4 only reduces
the atomic op count by half but introduces at lot of extra
complexity. It won't make a difference to the scalability of
workloads that hammer the btree block count because contention on
the internal counter spinlock will occur at close to the same
concurrency rate as would occur on an atomic counter.

Hence a per-cpu counter used in this manner seems like a premature
optimisation to me...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com