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

Eric Dumazet
2015-04-16T20:55:50Z

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

87.60% opensock [kernel.kallsyms] [k] _raw_spin_lock
1.57% opensock [kernel.kallsyms] [k] find_next_zero_bit
0.50% opensock [kernel.kallsyms] [k] memset_erms
0.44% opensock [kernel.kallsyms] [k] __alloc_fd
0.44% opensock [kernel.kallsyms] [k] tcp_close
0.43% opensock [kernel.kallsyms] [k] get_empty_filp
0.43% opensock [kernel.kallsyms] [k] kmem_cache_free
0.40% opensock [kernel.kallsyms] [k] free_block
0.34% opensock [kernel.kallsyms] [k] __close_fd
0.32% opensock [kernel.kallsyms] [k] sk_alloc
0.30% opensock [kernel.kallsyms] [k] _raw_spin_lock_bh
0.24% opensock [kernel.kallsyms] [k] inet_csk_destroy_sock
0.22% opensock [kernel.kallsyms] [k] kmem_cache_alloc
0.22% opensock opensock [.] __pthread_disable_asynccancel
0.21% opensock [kernel.kallsyms] [k] lockref_put_return
0.20% opensock [kernel.kallsyms] [k] filp_close

perf record -g ./opensock ; perf report --stdio

87.80% opensock [kernel.kallsyms] [k] _raw_spin_lock
|
--- _raw_spin_lock
|
|--52.70%-- __close_fd
| sys_close
| system_call_fastpath
| __libc_close
| |
| |--98.97%-- 0x0
| --1.03%-- [...]
|
|--46.41%-- __alloc_fd
| get_unused_fd_flags
| sock_map_fd
| sys_socket
| system_call_fastpath
| __socket
| |
| --100.00%-- 0x0
--0.89%-- [...]

1.54% opensock [kernel.kallsyms] [k] find_next_zero_bit
|