Re: [PATCH] fs.h: Optimize file struct to prevent false sharing

[Christian Brauner]

On Tue, May 30, 2023 at 06:55:49PM -0700, Eric Biggers wrote:
> On Tue, May 30, 2023 at 10:50:42AM +0200, Christian Brauner wrote:
> > On Mon, May 29, 2023 at 10:06:26PM -0400, chenzhiyin wrote:
> > > In the syscall test of UnixBench, performance regression occurred
> > > due to false sharing.
> > > 
> > > The lock and atomic members, including file::f_lock, file::f_count
> > > and file::f_pos_lock are highly contended and frequently updated
> > > in the high-concurrency test scenarios. perf c2c indentified one
> > > affected read access, file::f_op.
> > > To prevent false sharing, the layout of file struct is changed as
> > > following
> > > (A) f_lock, f_count and f_pos_lock are put together to share the
> > > same cache line.
> > > (B) The read mostly members, including f_path, f_inode, f_op are
> > > put into a separate cache line.
> > > (C) f_mode is put together with f_count, since they are used
> > > frequently at the same time.
> > > 
> > > The optimization has been validated in the syscall test of
> > > UnixBench. performance gain is 30~50%, when the number of parallel
> > > jobs is 16.
> > > 
> > > Signed-off-by: chenzhiyin <zhiyin.chen@xxxxxxxxx>
> > > ---
> > 
> > Sounds interesting, but can we see the actual numbers, please? 
> > So struct file is marked with __randomize_layout which seems to make
> > this whole reordering pointless or at least only useful if the
> > structure randomization Kconfig is turned off. Is there any precedence
> > to optimizing structures that are marked as randomizable?
> 
> Most people don't use CONFIG_RANDSTRUCT.  So it's still worth optimizing struct
> layouts for everyone else.

Ok, good to know.
We should still see actual numbers and the commit message should mention
that this interacts with __randomize_layout and why it's still useful.