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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Tejun Heo <tj@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] kernfs: release kernfs_mutex before the inode allocation
Date: Wed, 17 Nov 2021 07:44:44 +0100	[thread overview]
Message-ID: <YZSk3DECnnknOu5T@kroah.com> (raw)
In-Reply-To: <YZQkQcrldGFwqV/r@google.com>

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@kernel.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)?

> > 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?
> 
> https://developer.android.com/studio/command-line/perfetto
> https://perfetto.dev/docs/data-sources/cpu-scheduling

That is links to a tool, not a test we can run ourselves.

Or how about the output of that tool?

> Android has perfetto tracing system and can show where processes
> were stuck. This case was the lock since holder was in direct reclaim
> path.

Reclaim of what?  What is the interaction here with kernfs?  Normally
this filesystem is not on any "fast paths" that I know of.

More specifics would be nice :)

thanks,

greg k-h

  reply	other threads:[~2021-11-17  6:44 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-16 19:43 Minchan Kim
2021-11-16 19:49 ` Greg Kroah-Hartman
2021-11-16 21:36   ` Minchan Kim
2021-11-17  6:44     ` Greg Kroah-Hartman [this message]
2021-11-17  7:27       ` Minchan Kim
2021-11-17  7:39         ` Greg Kroah-Hartman
2021-11-17 21:43           ` Minchan Kim
2021-11-17 21:45         ` Tejun Heo
2021-11-17 22:13           ` Minchan Kim
2021-11-17 22:23             ` Tejun Heo
2021-11-18  1:55               ` Minchan Kim
2021-11-18 16:35                 ` Tejun Heo

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