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

[Karel Zak]

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@xxxxxxxxxx>
 http://karelzak.blogspot.com