Re: hackbench regression with 2.6.36-rc1

[Eric W. Biederman]

"Zhang, Yanmin" <yanmin_zhang@xxxxxxxxxxxxxxx> writes:

> Comparing with 2.6.35's result, hackbench (thread mode) has about
> 80% regression on dual-socket Nehalem machine and about 90% regression
> on 4-socket Tigerton machines.

That seems unfortunate.  Do you only show a regression in the pthread
hackbench test?  Do you show a regression when you use pipes?

Does the size of the regression very based on the number of loop
iterations?  I ask because it appears that on the last message the
sender will exit necessitating that the receiver put the senders pid.
Which should be atypical.

> Command to start hackbench:
> #./hackbench 100 thread 2000
>
> process mode has no such regression.
>
> Profiling shows:
> #perf top
>              samples  pcnt function                 DSO
>              _______ _____ ________________________ ________________________
>
>             74415.00 29.9% put_pid                  [kernel.kallsyms]       
>             38395.00 15.4% unix_stream_recvmsg      [kernel.kallsyms]       
>             34877.00 14.0% unix_stream_sendmsg      [kernel.kallsyms]       
>             25204.00 10.1% pid_vnr                  [kernel.kallsyms]       
>             21864.00  8.8% unix_scm_to_skb          [kernel.kallsyms]       
>             13637.00  5.5% cred_to_ucred            [kernel.kallsyms]       
>              6520.00  2.6% unix_destruct_scm        [kernel.kallsyms]       
>              4731.00  1.9% sock_alloc_send_pskb     [kernel.kallsyms]       
>
>
> With 2.6.35, perf doesn't show put_pid/pid_NR.

Yes.  2.6.35 is imperfect and can report the wrong pid in some
circumstances.  I am surprised nothing related to the reference count on
struct cred does not show up in your profiling traces.

You are performing statistical sampling so I don't believe the
percentage of hits per function is the same as the percentage of
time per function.

Given that we are talking about a scheduler benchmark that is
doing something rather artificial (inter thread communication via
sockets), I don't know that this case is worth worrying about.

> Alex Shi and I did a quick bisect and located below 2 patches.

That is a plausible result.  The atomic reference counts may
be causing you to ping pong cache lines between cpus.

Eric


> 1) commit 7361c36c5224519b258219fe3d0e8abc865d8134
> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Date:   Sun Jun 13 03:34:33 2010 +0000
>
>     af_unix: Allow credentials to work across user and pid namespaces.
>
>     In unix_skb_parms store pointers to struct pid and struct cred instead
>     of raw uid, gid, and pid values, then translate the credentials on
>     reception into values that are meaningful in the receiving processes
>     namespaces.
>
>
> 2) commit 257b5358b32f17e0603b6ff57b13610b0e02348f
> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Date:   Sun Jun 13 03:32:34 2010 +0000
>
>     scm: Capture the full credentials of the scm sender.
>
>     Start capturing not only the userspace pid, uid and gid values of the
>     sending process but also the struct pid and struct cred of the sending
>     process as well.
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