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    SubjectRe: [PATCH -tip 10/10] perf probe: Accessing members in data structures
    Frederic Weisbecker wrote:
    > On Wed, Mar 17, 2010 at 03:14:43PM -0400, Masami Hiramatsu wrote:
    >> Mark Wielaard wrote:
    >>> On Tue, 2010-03-16 at 18:06 -0400, Masami Hiramatsu wrote:
    >>>> Support accessing members in the data structures. With this,
    >>>> perf-probe accepts data-structure members(IOW, it now accepts
    >>>> dot '.' and arrow '->' operators) as probe arguemnts.
    >>>>
    >>>> e.g.
    >>>>
    >>>> ./perf probe --add 'schedule:44 rq->curr'
    >>>>
    >>>> ./perf probe --add 'vfs_read file->f_op->read file->f_path.dentry'
    >>>>
    >>>> Note that '>' can be interpreted as redirection in command-line.
    >>>
    >>> If you find that a problem then you can do like SystemTap does and allow
    >>> '.' in place of '->'. In the code you already use the
    >>> perf_probe_arg_field ref flag only to check that the DIE gives you the
    >>> same information. So you could just drop that and use any separator.
    >>> Then you decide based on whether you see a DW_TAG_pointer_type. This
    >>> gives the user some extra flexibility by letting them not having to care
    >>> about specifying extra type information already available elsewhere.
    >>
    >> Thanks, when designing this feature, I considered it too.
    >>
    >> Since perf probe already support displaying source code by --line option,
    >> users will read the probed code itself and try to probe it. In that case,
    >> I think they naturally use '.' and '->' as they read (they might try to
    >> copy & paste it).
    >>
    >> So, I think that it would be good to support both of '.' and '->' as
    >> they are used in the code, because it will not confuse users.
    >>
    >> Thank you,
    >
    >
    > Agreed.
    >
    > And lets people use what is common for them: expressions that follow
    > C rules in the context.
    >
    > And those who will be more familiar with perf probe will know they can
    > use the simplified "." based scheme.

    Hi Frederic, Steven,

    BTW, currently perf trace can't parse these 'data structure' type
    field yet.

    e.g.
    execute following commands;

    ./perf probe vfs_read 'file->f_mode'
    ./perf record -e probe:vfs_read -afR ls -l
    ./perf trace
    Warning: Error: expected ';' but read '->'
    Warning: failed to read event format for vfs_read

    Because perf trace expects that the field name should be a
    C-variable name. Moreover, if perf probe supports array element,
    it becomes more problematic, e.g. 'field:u8 array[8]'! oops!
    what does the field contain? an array with 8 elements? or
    8th element of 'array' array? :(

    Should I suppose that trace event format only accept C-style
    variable name for field?


    Thank you,

    --
    Masami Hiramatsu
    e-mail: mhiramat@redhat.com


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