Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

Mateusz Guzik
2015-04-16T22:00:39Z

On Thu, Apr 16, 2015 at 01:55:39PM -0700, Eric Dumazet wrote:
> On Thu, 2015-04-16 at 13:42 -0700, Eric Dumazet wrote:
> > On Thu, 2015-04-16 at 19:09 +0100, Al Viro wrote:
> > > On Thu, Apr 16, 2015 at 02:16:31PM +0200, Mateusz Guzik wrote:
> > > > @@ -165,8 +165,10 @@ static int expand_fdtable(struct files_struct *files, int nr)
> > > > cur_fdt = files_fdtable(files);
> > > > if (nr >= cur_fdt->max_fds) {
> > > > /* Continue as planned */
> > > > + write_seqcount_begin(&files->fdt_seqcount);
> > > > copy_fdtable(new_fdt, cur_fdt);
> > > > rcu_assign_pointer(files->fdt, new_fdt);
> > > > + write_seqcount_end(&files->fdt_seqcount);
> > > > if (cur_fdt != &files->fdtab)
> > > > call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
> > >
> > > Interesting. AFAICS, your test doesn't step anywhere near that path,
> > > does it? So basically you never hit the retries during that...
> >
> > Right, but then the table is almost never changed for a given process,
> > as we only increase it by power of two steps.
> >
> > (So I scratch my initial comment, fdt_seqcount is really mostly read)
>
> I tested Mateusz patch with my opensock program, mimicking a bit more
> what a server does (having lot of sockets)
>
> 24 threads running, doing close(randomfd())/socket() calls like crazy.
>
> Before patch :
>
> # time ./opensock
>
> real 0m10.863s
> user 0m0.954s
> sys 2m43.659s
>
>
> After patch :
>
> # time ./opensock
>
> real 0m9.750s
> user 0m0.804s
> sys 2m18.034s
>
> So this is an improvement for sure, but not massive.
>
> perf record ./opensock ; report
>
> 87.80% opensock [kernel.kallsyms] [k] _raw_spin_lock
> |--52.70%-- __close_fd
> |--46.41%-- __alloc_fd

My crap benchmark is here: http://people.redhat.com/~mguzik/pipebench.c
(compile with -pthread, run with -s 10 -n 16 for 10 second test + 16
threads)

As noted earlier it tends to go from rougly 300k ops/s to 400.

The fundamental problem here seems to be this pesky POSIX requirement of
providing the lowest possible fd on each allocation (as a side note
Linux breaks this with parallel fd allocs, where one of these backs off
the reservation, not that I believe this causes trouble).

Ideally a process-wide switch could be implemented (e.g.
prctl(SCRATCH_LOWEST_FD_REQ)) which would grant the kernel the freedom
to return any fd it wants, so it would be possible to have fd ranges
per thread and the like.

Having only a O_SCRATCH_POSIX flag passed to syscalls would still leave
close() as a bottleneck.

In the meantime I consider the approach taken in my patch as an ok
temporary improvement.

--
Mateusz Guzik