• Tim Chen's avatar
    Scm: Remove unnecessary pid & credential references in Unix socket's send and receive path · 0856a304
    Tim Chen authored
    Patch series 109f6e39..7361c36c back in 2.6.36 added functionality to
    allow credentials to work across pid namespaces for packets sent via
    UNIX sockets.  However, the atomic reference counts on pid and
    credentials caused plenty of cache bouncing when there are numerous
    threads of the same pid sharing a UNIX socket.  This patch mitigates the
    problem by eliminating extraneous reference counts on pid and
    credentials on both send and receive path of UNIX sockets. I found a 2x
    improvement in hackbench's threaded case.
    
    On the receive path in unix_dgram_recvmsg, currently there is an
    increment of reference count on pid and credentials in scm_set_cred.
    Then there are two decrement of the reference counts.  Once in scm_recv
    and once when skb_free_datagram call skb->destructor function
    unix_destruct_scm.  One pair of increment and decrement of ref count on
    pid and credentials can be eliminated from the receive path.  Until we
    destroy the skb, we already set a reference when we created the skb on
    the send side.
    
    On the send path, there are two increments of ref count on pid and
    credentials, once in scm_send and once in unix_scm_to_skb.  Then there
    is a decrement of the reference counts in scm_destroy's call to
    scm_destroy_cred at the end of unix_dgram_sendmsg functions.   One pair
    of increment and decrement of the reference counts can be removed so we
    only need to increment the ref counts once.
    
    By incorporating these changes, for hackbench running on a 4 socket
    NHM-EX machine with 40 cores, the execution of hackbench on
    50 groups of 20 threads sped up by factor of 2.
    
    Hackbench command used for testing:
    ./hackbench 50 thread 2000
    Signed-off-by: default avatarTim Chen <tim.c.chen@linux.intel.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    0856a304
scm.h 3.52 KB