Messages in this thread | | | Date | Sat, 20 Mar 2010 00:20:12 -0400 | From | Masami Hiramatsu <> | Subject | Re: [PATCH -tip 10/10] perf probe: Accessing members in data structures |
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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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