linux-kernel - Re: [RFC PATCH] getvalues(2) prototype

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Date:   Fri, 25 Mar 2022 10:25:53 +0100
From:   Karel Zak <kzak@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Miklos Szeredi <miklos@...redi.hu>, Theodore Ts'o <tytso@....edu>,
        Christian Brauner <brauner@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getvalues(2) prototype

On Fri, Mar 25, 2022 at 09:54:21AM +0100, Greg KH wrote:
> On Fri, Mar 25, 2022 at 09:46:46AM +0100, Karel Zak wrote:
> > On Thu, Mar 24, 2022 at 09:44:38AM +0100, Miklos Szeredi wrote:
> > > > If so, have you benchmarked lsof using this new interface?
> > > 
> > > Not yet.  Looked yesterday at both lsof and procps source code, and
> > > both are pretty complex and not easy to plug in a new interface.   But
> > > I've not yet given up...
> > 
> > I can imagine something like getvalues(2) in lsblk (based on /sys) or
> > in lsfd (based on /proc; lsof replacement). The tools have defined set
> > of information to read from kernel, so gather all the requests to the
> > one syscall for each process or block device makes sense and it will
> > dramatically reduce number of open+read+close syscalls.
> 
> And do those open+read+close syscalls actually show up in measurements?
> 
> Again, I tried to find a real-world application that turning those 3
> into 1 would matter, and I couldn't.  procps had no decreased system
> load that I could notice.  I'll mess with lsof but that's really just a
> stress-test, not anything that is run all the time, right?


Right, the speed of ps(1) or lsof(1) is not important. IMHO the current
discussion about getvalues() goes in wrong direction :-)  

I guess the primary motivation is not to replace open+read+close, but
provide to userspace something usable to get information from mount
table, because the current /proc/#/mountinfo and notification by
poll() is horrible.

Don't forget that the previous attempt was fsinfo() from David Howells
(unfortunately, it was too complex and rejected by Linus).

> And as others have said, using io_uring() would also solve the 3 syscall
> issue, but no one seems to want to convert these tools to use that,
> which implies that it's not really an issue for anyone :)

OK, I'll think about it :-)

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com