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

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Message-ID: <YZQkQcrldGFwqV/r@google.com>
Date:   Tue, 16 Nov 2021 13:36:01 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.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 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.

> 
> 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

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