Re: more dput lock contentions in 2.6.38-rc?

[Nick Piggin]

On Tue, Jan 25, 2011 at 11:35 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
> Hi,
> we are testing dbench benchmark and see big drop of 2.6.38-rc compared
> to 2.6.37 in several machines with 2 sockets or 4 sockets. We have 12
> disks mount to /mnt/stp/dbenchdata/sd*/ and dbench runs against data of
> the disks. According to perf, we saw more lock contentions:
> In 2.6.37: 13.00%        dbench  [kernel.kallsyms]   [k] _raw_spin_lock
> In 2.6.38-rc: 69.45%        dbench  [kernel.kallsyms]   [k]_raw_spin_lock
> -     69.45%        dbench  [kernel.kallsyms]   [k] _raw_spin_lock
>   - _raw_spin_lock
>      - 48.41% dput
>         - 61.17% path_put
>            - 60.47% do_path_lookup
>               + 53.18% user_path_at
>               + 42.13% do_filp_open
>               + 4.69% user_path_parent

What filesystems are mounted on the path?


>            - 35.56% d_path
>                 seq_path
>                 show_vfsmnt
>                 seq_read
>                 vfs_read
>                 sys_read
>                 system_call_fastpath
>                 __GI___libc_read

This guy is from glibc's statvfs call that dbench uses. It
parses /proc/mounts for mount flags which is racy (and
not a good idea to do with any frequency).

A patch went into the kernel that allows glibc to get the
flags directly. Not sure about glibc status, I imagine it
will get there in another decade or two... Can you try
commenting it out of dbench source code?


>            + 2.17% do_filp_open
>            + 1.72% mounts_release
>         + 38.69% link_path_walk
>      + 30.21% path_get
>      + 19.08% nameidata_drop_rcu
>      + 0.83% __d_lookup
> it appears there are heavy lock contention when dput release '/', 'mnt',
> 'stp', 'dbenchdata' and 'proc' when dbench is running.
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