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

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Date:   Wed, 17 Nov 2021 08:39:44 +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 11:27:56PM -0800, Minchan Kim wrote:
> On Wed, Nov 17, 2021 at 07:44:44AM +0100, Greg Kroah-Hartman wrote:
> > On Tue, Nov 16, 2021 at 01:36:01PM -0800, Minchan Kim wrote:
> > > On Tue, Nov 16, 2021 at 08:49:46PM +0100, Greg Kroah-Hartman wrote:
> > > > 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?
> > > 
> > > A app launching since it was dropping the frame since the
> > > latency was too long.
> > 
> > How does running a program interact with kernfs filesystems?  Which
> > one(s)?
> 
> A app launching involves dma_buf exports which creates kobject
> and add it to the kernfs with down_write - kernfs_add_one.

I thought the "create a dma_buf kobject" kernel change was fixed up to
not do that anymore as that was a known performance issue.

Creating kobjects should NOT be on a fast path, if they are, that needs
to be fixed.

> At the same time in other CPU, a random process was accessing
> sysfs and the kernfs_iop_lookup was already hoding the kernfs_rwsem
> and ran under direct reclaim patch due to alloc_inode in
> kerfs_get_inode.

What program is constantly hitting sysfs?  sysfs is not for
performance-critical things, right?

> Therefore, the app is stuck on the lock and lose frames so enduser
> sees the jank.

But how does this patch work around it?  It seems like you are
special-casing the kobject creation path only.

And is this the case really on 5.15?  I thought the kernfs locks were
broken up again to not cause this problem in 5.14 or so.

thanks,

greg k-h