linux-kernel - Re: [RFC PATCH] kernfs: release kernfs

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Date:   Tue, 16 Nov 2021 20:49:46 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] kernfs: release kernfs_mutex before the inode
 allocation

On Tue, Nov 16, 2021 at 11:43:17AM -0800, Minchan Kim wrote:
> The kernfs implementation has big lock granularity(kernfs_rwsem) so
> every kernfs-based(e.g., sysfs, cgroup, dmabuf) fs are able to compete
> the lock. Thus, if one of userspace goes the sleep under holding
> the lock for a long time, rest of them should wait it. A example is
> the holder goes direct reclaim with the lock since it needs memory
> allocation. Let's fix it at common technique that release the lock
> and then allocate the memory. Fortunately, kernfs looks like have
> an refcount so I hope it's fine.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  fs/kernfs/dir.c             | 14 +++++++++++---
>  fs/kernfs/inode.c           |  2 +-
>  fs/kernfs/kernfs-internal.h |  1 +
>  3 files changed, 13 insertions(+), 4 deletions(-)

What workload hits this lock to cause it to be noticable?

There was a bunch of recent work in this area to make this much more
fine-grained, and the theoritical benchmarks that people created (adding
10s of thousands of scsi disks at boot time) have gotten better.

But in that work, no one could find a real benchmark or use case that
anyone could even notice this type of thing.  What do you have that
shows this?
thanks,

greg k-h