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

Greg Kroah-Hartman
2020-03-04T16:49:22Z

On Wed, Mar 04, 2020 at 04:22:41PM +0100, Karel Zak wrote:
> 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

As a "real-world" test, would you recommend me converting one of the
above tools to my implementation of readfile to see how/if it actually
makes sense, or do you have some other tool you would rather see me try?

thanks,

greg k-h