On Thu, Oct 31, 2019 at 08:43:35AM +1100, Dave Chinner wrote:
> On Wed, Oct 30, 2019 at 10:25:17AM -0700, Darrick J. Wong wrote:
> > On Wed, Oct 09, 2019 at 02:21:02PM +1100, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > >
> > > The buffer cache shrinker frees more than just the xfs_buf slab
> > > objects - it also frees the pages attached to the buffers. Make sure
> > > the memory reclaim code accounts for this memory being freed
> > > correctly, similar to how the inode shrinker accounts for pages
> > > freed from the page cache due to mapping invalidation.
> > >
> > > We also need to make sure that the mm subsystem knows these are
> > > reclaimable objects. We provide the memory reclaim subsystem with a
> > > a shrinker to reclaim xfs_bufs, so we should really mark the slab
> > > that way.
> > >
> > > We also have a lot of xfs_bufs in a busy system, spread them around
> > > like we do inodes.
> > >
> > > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > > ---
> > > fs/xfs/xfs_buf.c | 6 +++++-
> > > 1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > > index e484f6bead53..45b470f55ad7 100644
> > > --- a/fs/xfs/xfs_buf.c
> > > +++ b/fs/xfs/xfs_buf.c
> > > @@ -324,6 +324,9 @@ xfs_buf_free(
> > >
> > > __free_page(page);
> > > }
> > > + if (current->reclaim_state)
> > > + current->reclaim_state->reclaimed_slab +=
> > > + bp->b_page_count;
> >
> > Hmm, ok, I see how ZONE_RECLAIM and reclaimed_slab fit together.
> >
> > > } else if (bp->b_flags & _XBF_KMEM)
> > > kmem_free(bp->b_addr);
> > > _xfs_buf_free_pages(bp);
> > > @@ -2064,7 +2067,8 @@ int __init
> > > xfs_buf_init(void)
> > > {
> > > xfs_buf_zone = kmem_zone_init_flags(sizeof(xfs_buf_t), "xfs_buf",
> > > - KM_ZONE_HWALIGN, NULL);
> > > + KM_ZONE_HWALIGN | KM_ZONE_SPREAD | KM_ZONE_RECLAIM,
> >
> > I guess I'm fine with ZONE_SPREAD too, insofar as it only seems to apply
> > to a particular "use another node" memory policy when slab is in use.
> > Was that your intent?
>
> It's more documentation than anything - that we shouldn't be piling
> these structures all on to one node because that can have severe
> issues with NUMA memory reclaim algorithms. i.e. the xfs-buf
> shrinker sets SHRINKER_NUMA_AWARE, so memory pressure on a single
> node can reclaim all the xfs-bufs on that node without touching any
> other node.
>
> That means, for example, if we instantiate all the AG header buffers
> on a single node (e.g. like we do at mount time) then memory
> pressure on that one node will generate IO stalls across the entire
> filesystem as other nodes doing work have to repopulate the buffer
> cache for any allocation for freeing of space/inodes..
>
> IOWs, for large NUMA systems using cpusets this cache should be
> spread around all of memory, especially as it has NUMA aware
> reclaim. For everyone else, it's just documentation that improper
> cgroup or NUMA memory policy could cause you all sorts of problems
> with this cache.
>
> It's worth noting that SLAB_MEM_SPREAD is used almost exclusively in
> filesystems for inode caches largely because, at the time (~2006),
> the only reclaimable cache that could grow to any size large enough
> to cause problems was the inode cache. It's been cargo-culted ever
> since, whether it is needed or not (e.g. ceph).
>
> In the case of the xfs_bufs, I've been running workloads recently
> that cache several million xfs_bufs and only a handful of inodes
> rather than the other way around. If we spread inodes because
> caching millions on a single node can cause problems on large NUMA
> machines, then we also need to spread xfs_bufs...
Hmm, could we capture this as a comment somewhere?
--D
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
