linux-kernel - Re: [PATCH 2/2] vfs: grab the lock instead of blocking in __fd

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Date:   Wed, 4 Oct 2017 07:58:35 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Mateusz Guzik <mguzik@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, mszeredi@...hat.com,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] vfs: grab the lock instead of blocking in
 __fd_install during resizing

On Wed, Oct 04, 2017 at 07:00:40AM -0700, Eric Dumazet wrote:
> On Wed, Oct 4, 2017 at 6:55 AM, Matthew Wilcox <willy@...radead.org> wrote:
> > Any chance you could review the patches from Sandhya that make this entire
> > codepath obsolete?
> >
> > https://lkml.org/lkml/2017/4/29/20
> >
> 
> Hmm...
> 
> 18 files changed, 578 insertions(+), 585 deletions(-)
> 
> Frankly I need to be convinced with solid performance numbers before I
> am taking a look at this series.

I was hoping you'd help us get some solid performance numbers ... you
seem to have workloads available to you that help find weaknesses in
implementations.

The number of lines inserted is a bit of a red herring.  Over 100 are in
the test suite (you surely aren't going to review those) and another ~300
are adding enhancements to the IDR & radix tree that should be useful
for other users (eg I think I have a way to speed up writing out dirty
pages by using get_tag_batch()).

> I do not believe an IDR will be faster than current implementation, so
> I am not quite convinced at this moment.

I don't think it should be significantly different in performance.  Let's
look at the layout of data for a typical bash shell (fds 0-2 and 255 open).

Current implementation:

files_struct -> fdt -> fd -> struct file

IDR:

files_struct -> radix node (shift 6) -> radix node (shift 0) -> struct file

In either case, it's the same number of dependent loads.  It'll start
to look worse for the radix tree above 4096 open fds in a given process.