netdev - Re: [PATCH v2 net-next] af_unix: dont send SCM

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Date:	Mon, 19 Sep 2011 14:39:58 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, zheng.z.yan@...el.com,
	yanzheng@...n.com, netdev@...r.kernel.org, sfr@...b.auug.org.au,
	jirislaby@...il.com, sedat.dilek@...il.com, alex.shi@...el.com,
	Valdis.Kletnieks@...edu
Subject: Re: [PATCH v2 net-next] af_unix: dont send SCM_CREDENTIALS by
 default

On Mon, 2011-09-19 at 17:52 +0200, Eric Dumazet wrote:
> Since commit 7361c36c5224 (af_unix: Allow credentials to work across
> user and pid namespaces) af_unix performance dropped a lot.
> 
> This is because we now take a reference on pid and cred in each write(),
> and release them in read(), usually done from another process,
> eventually from another cpu. This triggers false sharing.
> 
> # Events: 154K cycles
> #
> # Overhead  Command       Shared Object        Symbol
> # ........  .......  ..................  .........................
> #
>     10.40%  hackbench  [kernel.kallsyms]   [k] put_pid
>      8.60%  hackbench  [kernel.kallsyms]   [k] unix_stream_recvmsg
>      7.87%  hackbench  [kernel.kallsyms]   [k] unix_stream_sendmsg
>      6.11%  hackbench  [kernel.kallsyms]   [k] do_raw_spin_lock
>      4.95%  hackbench  [kernel.kallsyms]   [k] unix_scm_to_skb
>      4.87%  hackbench  [kernel.kallsyms]   [k] pid_nr_ns
>      4.34%  hackbench  [kernel.kallsyms]   [k] cred_to_ucred
>      2.39%  hackbench  [kernel.kallsyms]   [k] unix_destruct_scm
>      2.24%  hackbench  [kernel.kallsyms]   [k] sub_preempt_count
>      1.75%  hackbench  [kernel.kallsyms]   [k] fget_light
>      1.51%  hackbench  [kernel.kallsyms]   [k]
> __mutex_lock_interruptible_slowpath
>      1.42%  hackbench  [kernel.kallsyms]   [k] sock_alloc_send_pskb
> 
> 
> This patch includes SCM_CREDENTIALS information in a af_unix message/skb
> only if requested by the sender, [man 7 unix for details how to include
> ancillary data using sendmsg() system call]
> 
> Note: This might break buggy applications that expected SCM_CREDENTIAL
> from an unaware write() system call, and receiver not using SO_PASSCRED
> socket option.
> 
> If SOCK_PASSCRED is set on source or destination socket, we still
> include credentials for mere write() syscalls.
> 
> Performance boost in hackbench : more than 50% gain on a 16 thread
> machine (2 quad-core cpus, 2 threads per core)
> 
> hackbench 20 thread 2000
> 
> 4.228 sec instead of 9.102 sec
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---

Do we have to worry about the case where peer socket changes its flag
to SOCK_PASSCRED while packets are in flight?  If there isn't such
pathological use case, the patch looks fine to me.

Acked-by: Tim Chen <tim.c.chen@...ux.intel.com>

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