netdev - Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect for sockets in accept(3)

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Date:	Sat, 31 Oct 2015 15:34:44 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	David Miller <davem@...emloft.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Network Development <netdev@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect
 for sockets in accept(3)

On Sat, 2015-10-31 at 14:23 -0700, Linus Torvalds wrote:

> Mind testing something really stupid, and making the __clear_bit() in
> __clear_close_on_exec() conditiona, something like this:
> 
>      static inline void __clear_close_on_exec(int fd, struct fdtable *fdt)
>      {
>     -       __clear_bit(fd, fdt->close_on_exec);
>     +       if (test_bit(fd, fdt->close_on_exec)
>     +               __clear_bit(fd, fdt->close_on_exec);
>      }
> 
> and see if it makes a difference.

It does ;)

About 4 % qps increase

3 runs : 
lpaa24:~# taskset ff0ff ./opensock -t 16 -n 10000000 -l 10
total = 4176651
total = 4178012
total = 4105226

instead of :
total = 3910620
total = 3874567
total = 3971028

Perf profile :

    69.12%       opensock  opensock             [.] memset                                       
                 |
                 --- memset

    12.37%       opensock  [kernel.kallsyms]    [k] queued_spin_lock_slowpath                    
                 |
                 --- queued_spin_lock_slowpath
                    |          
                    |--99.99%-- _raw_spin_lock
                    |          |          
                    |          |--51.99%-- __close_fd
                    |          |          sys_close
                    |          |          entry_SYSCALL_64_fastpath
                    |          |          __libc_close
                    |          |          |          
                    |          |           --100.00%-- 0x0
                    |          |          
                    |          |--47.79%-- __alloc_fd
                    |          |          get_unused_fd_flags
                    |          |          sock_map_fd
                    |          |          sys_socket
                    |          |          entry_SYSCALL_64_fastpath
                    |          |          __socket
                    |          |          |          
                    |          |           --100.00%-- 0x0
                    |           --0.21%-- [...]
                     --0.01%-- [...]

     1.92%       opensock  [kernel.kallsyms]    [k] _find_next_bit.part.0                        
                 |
                 --- _find_next_bit.part.0
                    |          
                    |--66.93%-- find_next_zero_bit
                    |          __alloc_fd
                    |          get_unused_fd_flags
                    |          sock_map_fd
                    |          sys_socket
                    |          entry_SYSCALL_64_fastpath
                    |          __socket
                    |          
                     --33.07%-- __alloc_fd
                               get_unused_fd_flags
                               sock_map_fd
                               sys_socket
                               entry_SYSCALL_64_fastpath
                               __socket
                               |          
                                --100.00%-- 0x0

     1.63%       opensock  [kernel.kallsyms]    [k] _raw_spin_lock                               
                 |
                 --- _raw_spin_lock
                    |          
                    |--28.66%-- get_unused_fd_flags
                    |          sock_map_fd


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