linux-kernel - Re: [PATCH RFC 6/7] libfs: Convert simple directory offsets to use a Maple Tree

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Date: Fri, 16 Feb 2024 16:15:08 +0100
From: Christian Brauner <brauner@...nel.org>
To: Chuck Lever <chuck.lever@...cle.com>
Cc: Jan Kara <jack@...e.cz>, Chuck Lever <cel@...nel.org>, 
	viro@...iv.linux.org.uk, hughd@...gle.com, akpm@...ux-foundation.org, 
	Liam.Howlett@...cle.com, oliver.sang@...el.com, feng.tang@...el.com, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, maple-tree@...ts.infradead.org, 
	linux-mm@...ck.org, lkp@...el.com
Subject: Re: [PATCH RFC 6/7] libfs: Convert simple directory offsets to use a
 Maple Tree

On Thu, Feb 15, 2024 at 08:45:33AM -0500, Chuck Lever wrote:
> On Thu, Feb 15, 2024 at 02:06:01PM +0100, Jan Kara wrote:
> > On Tue 13-02-24 16:38:01, Chuck Lever wrote:
> > > From: Chuck Lever <chuck.lever@...cle.com>
> > > 
> > > Test robot reports:
> > > > kernel test robot noticed a -19.0% regression of aim9.disk_src.ops_per_sec on:
> > > >
> > > > commit: a2e459555c5f9da3e619b7e47a63f98574dc75f1 ("shmem: stable directory offsets")
> > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > > 
> > > Feng Tang further clarifies that:
> > > > ... the new simple_offset_add()
> > > > called by shmem_mknod() brings extra cost related with slab,
> > > > specifically the 'radix_tree_node', which cause the regression.
> > > 
> > > Willy's analysis is that, over time, the test workload causes
> > > xa_alloc_cyclic() to fragment the underlying SLAB cache.
> > > 
> > > This patch replaces the offset_ctx's xarray with a Maple Tree in the
> > > hope that Maple Tree's dense node mode will handle this scenario
> > > more scalably.
> > > 
> > > In addition, we can widen the directory offset to an unsigned long
> > > everywhere.
> > > 
> > > Suggested-by: Matthew Wilcox <willy@...radead.org>
> > > Reported-by: kernel test robot <oliver.sang@...el.com>
> > > Closes: https://lore.kernel.org/oe-lkp/202309081306.3ecb3734-oliver.sang@intel.com
> > > Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
> > 
> > OK, but this will need the performance numbers.
> 
> Yes, I totally concur. The point of this posting was to get some
> early review and start the ball rolling.
> 
> Actually we expect roughly the same performance numbers now. "Dense
> node" support in Maple Tree is supposed to be the real win, but
> I'm not sure it's ready yet.

I keep repeating this but we need a better way to request performance
tests for specific series/branches. Maybe I can add a vfs.perf branch
where we can put patches that we suspect have positive/negative perf
impact and that perf bot can pull that in. I know, that's a fuzzy
boundary but for stuff like this where we already know that there's a
perf impact that's important for us it would really help.

Because it is royally annoying to get a perf regression report after a
patch has been in -next for a long time already and the merge window is
coming up or we already merged that stuff.