Re: [RFC] proc: report open files as size in stat() for /proc/pid/fd

[Ivan Babrou]

On Fri, Sep 16, 2022 at 5:01 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> (cc's added)
>
> On Fri, 16 Sep 2022 16:08:52 -0700 Ivan Babrou <ivan@xxxxxxxxxxxxxx> wrote:
>
> > Many monitoring tools include open file count as a metric. Currently
> > the only way to get this number is to enumerate the files in /proc/pid/fd.
> >
> > The problem with the current approach is that it does many things people
> > generally don't care about when they need one number for a metric.
> > In our tests for cadvisor, which reports open file counts per cgroup,
> > we observed that reading the number of open files is slow. Out of 35.23%
> > of CPU time spent in `proc_readfd_common`, we see 29.43% spent in
> > `proc_fill_cache`, which is responsible for filling dentry info.
> > Some of this extra time is spinlock contention, but it's a contention
> > for the lock we don't want to take to begin with.
> >
> > We considered putting the number of open files in /proc/pid/stat.
> > Unfortunately, counting the number of fds involves iterating the fdtable,
> > which means that it might slow down /proc/pid/stat for processes
> > with many open files. Instead we opted to put this info in /proc/pid/fd
> > as a size member of the stat syscall result. Previously the reported
> > number was zero, so there's very little risk of breaking anything,
> > while still providing a somewhat logical way to count the open files.
>
> Documentation/filesystems/proc.rst would be an appropriate place to
> document this ;)

I am more than happy to add the docs after there's a confirmation that
this is an appropriate approach to expose this information. I probably
should've mentioned this explicitly, that's on me. There are two
alternative approaches at the bottom of my original email that might
be considered.