On Mon, Aug 15, 2016 at 04:48:36PM -0700, Linus Torvalds wrote:
> On Mon, Aug 15, 2016 at 4:20 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > None of this code is all that new, which is annoying. This must have
> > gone on forever,
> 
> ... ooh.
> 
> Wait, I take that back.
> 
> We actually have some very recent changes that I didn't even think
> about that went into this very merge window.
....
> Mel? The issue is that Dave Chinner is seeing some nasty spinlock
> contention on "mapping->tree_lock":
> 
> >   31.18%  [kernel]  [k] __pv_queued_spin_lock_slowpath
> 
> and one of the main paths is this:
> 
> >    - 30.29% kswapd
> >       - 30.23% shrink_node
> >          - 30.07% shrink_node_memcg.isra.75
> >             - 30.15% shrink_inactive_list
> >                - 29.49% shrink_page_list
> >                   - 22.79% __remove_mapping
> >                      - 22.27% _raw_spin_lock_irqsave
> >                           __pv_queued_spin_lock_slowpath
> 
> so there's something ridiculously bad going on with a fairly simple benchmark.
> 
> Dave's benchmark is literally just a "write a new 48GB file in
> single-page chunks on a 4-node machine". Nothing odd - not rewriting
> files, not seeking around, no nothing.
> 
> You can probably recreate it with a silly
> 
>   dd bs=4096 count=$((12*1024*1024)) if=/dev/zero of=bigfile
> 
> although Dave actually had something rather fancier, I think.

16p, 16GB RAM, fake_numa=4. Overwrite a 47GB file on a 48GB
filesystem:

# mkfs.xfs -f -d size=48g /dev/vdc
# mount /dev/vdc /mnt/scratch
# xfs_io -f -c "pwrite 0 47g" /mnt/scratch/fooey

Wait for memory to fill and reclaim to kick in, then look at the
profile. If you run it a second time, reclaim kicks in straight
away.

It's not the new code in 4.8 - it reproduces on 4.7 just fine, and
probably will reproduce all the way back to when the memcg-aware
writeback code was added....

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