linux-kernel - Re: [kernfs] ea7c5fc39a: stress-ng.stream.ops_per

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Date:   Sun, 07 Jun 2020 09:13:08 +0800
From:   Ian Kent <raven@...maw.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel test robot <rong.a.chen@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, Tejun Heo <tj@...nel.org>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        lkp@...ts.01.org
Subject: Re: [kernfs] ea7c5fc39a: stress-ng.stream.ops_per_sec 11827.2%
 improvement

On Sat, 2020-06-06 at 20:18 +0200, Greg Kroah-Hartman wrote:
> On Sat, Jun 06, 2020 at 11:52:16PM +0800, kernel test robot wrote:
> > Greeting,
> > 
> > FYI, we noticed a 11827.2% improvement of stress-
> > ng.stream.ops_per_sec due to commit:
> > 
> > 
> > commit: ea7c5fc39ab005b501e0c7666c29db36321e4f74 ("[PATCH 1/4]
> > kernfs: switch kernfs to use an rwsem")
> > url: 
> > https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20200525-134849
> > 
> 
> Seriously?  That's a huge performance increase, and one that feels
> really odd.  Why would a stress-ng test be touching sysfs?

That is unusually high even if there's a lot of sysfs or kernfs
activity and that patch shouldn't improve VFS path walk contention
very much even if it is present.

Maybe I've missed something, and the information provided doesn't
seem to be quite enough to even make a start on it.

That's going to need some analysis which, for my part, will need to
wait probably until around rc1 time frame to allow me to get through
the push down stack (reactive, postponed due to other priorities) of
jobs I have in order to get back to the fifo queue (longer term tasks,
of which this is one) list of jobs I need to do as well, ;)

Please, kernel test robot, more information about this test and what
it's doing.

Ian