linux-kernel - Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd

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Date:	Thu, 16 Apr 2015 13:55:39 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Mateusz Guzik <mguzik@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install

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                 
            |





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