linux-kernel - Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

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Date:   Wed, 4 Mar 2020 16:22:41 +0100
From:   Karel Zak <kzak@...hat.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver
 #17]

On Wed, Mar 04, 2020 at 10:01:33AM +0800, Ian Kent wrote:
> On Tue, 2020-03-03 at 14:03 +0100, Greg Kroah-Hartman wrote:
> > Actually, I like this idea (the syscall, not just the unlimited
> > beers).
> > Maybe this could make a lot of sense, I'll write some actual tests
> > for
> > it now that syscalls are getting "heavy" again due to CPU vendors
> > finally paying the price for their madness...
> 
> The problem isn't with open->read->close but with the mount info.
> changing between reads (ie. seq file read takes and drops the
> needed lock between reads at least once).

readfile() is not reaction to mountinfo. 

The motivation is that we have many places with trivial
open->read->close for very small text files due to /sys and /proc. The
current way how kernel delivers these small strings to userspace seems
pretty inefficient if we can do the same by one syscall.

    Karel

$ strace -e openat,read,close -c ps aux
...
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 43.32    0.004190           4       987           read
 31.42    0.003039           3       844         4 openat
 25.26    0.002443           2       842           close
------ ----------- ----------- --------- --------- ----------------
100.00    0.009672                  2673         4 total

$ strace -e openat,read,close -c lsns
...
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 39.95    0.001567           2       593           openat
 30.93    0.001213           2       597           close
 29.12    0.001142           3       365           read
------ ----------- ----------- --------- --------- ----------------
100.00    0.003922                  1555           total


$ strace -e openat,read,close -c lscpu
...
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 44.67    0.001480           7       189        52 openat
 34.77    0.001152           6       180           read
 20.56    0.000681           4       140           close
------ ----------- ----------- --------- --------- ----------------
100.00    0.003313                   509        52 total


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