Messages in this thread | | | Date | Thu, 12 Jun 2008 13:05:52 -0400 | From | Masami Hiramatsu <> | Subject | Re: Kernel marker has no performance impact on ia64. |
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Hi,
Frank Ch. Eigler wrote: > Hi - > > On Thu, Jun 12, 2008 at 12:16:35PM -0400, Masami Hiramatsu wrote: >> [...] >>> Think this through. How should systemtap (or another user-space >>> separate-compiled tool like lttng) do this exactly? >>> [...] >>> (d) or another way? >> use a lookup table. we can expect that the marking points which >> regularly inserted in the upstream kernel are stable(not so >> frequently change). In that case, we can easily maintain >> a lookup table which has pairs of format strings like as >> "sched_switch(struct task_struct * next, struct task_struct * prev)":"next %p prev %p" >> out of tree. Thus, you can use the printf-style format parser. > > That's an interesting idea, but errors in this table would themselves > only be caught at C compilation time.
Hmm, why would you think so? I think if we can't find corresponding entry from the lookup table, it becomes an error.
> Worse, it does nothing helpful > for actually pulling out the next/prev fields of interest. Remember, > real tracing users don't care so much about the task_struct pointers, > but about observable things like PIDs. Systemtap would be back to the > debuginfo or C-header-guessing/parsing job (or embedded-C, yuck).
Yeah, but that is same as previous marker. It depends on what parameter the kernel pass to the marker. I mean, the parameter issue is not an issue of the marker framework, but a discussion point of marking points.
> This is another reason why markers are a nice solution. They allow > passing of actual useful values: not just the %p pointers but the most > interesting derived values (e.g., prev->pid). And they do this > *efficiently* - in out-of-line code that imposes no measurable > overhead in the normal case..
Even if you use trace_mark() markers, you have to post a kernel patch which passes the prev->pid to the marking point and to discuss it. for example, DEFINE_TRACE(sched_switch, (int prev_pid, int next_pid), prev_pid, next_pid)
But it might not so general, we have to discuss what parameters are enough good for each marking point.
> > > - FChE
Thank you,
-- Masami Hiramatsu
Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division
e-mail: mhiramat@redhat.com
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