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Re: [patch 1/3] Trace code and documentation

Andi Kleen wrote:
On Thu, Oct 04, 2007 at 12:19:35PM -0700, David Wilder wrote:
Andi Kleen wrote:
"David J. Wilder" <> writes:
@@ -0,0 +1,160 @@
+Trace Setup and Control
+In the kernel, the trace interface provides a simple mechanism for
+starting and managing data channels (traces) to user space.
Wasn't relayfs supposed to do that already? Why do you need another
wrapper around it?
The code in trace is exactly what all the current users of relay do. Therefor trace reduces the duplication of code.

If everybody does this then the code should be just put into relayfs?

I disagree, I keeping the code separate (layering if you will) makes it easer to use and maintain.

Is this also really still faster than a printk below log level
(without console driver overhead). If not then why not just
use printk?
Are you arguing against relayfs or trace? Trace just makes relayfs easer to use. I think relayfs can stand up for it's self.

I'm arguing against complicated trace mechanisms that are not fast.

What makes trace complicated? It is just, open ,start/stop, close. I can't see how an trace API could be any simpler.

At some point when I looked at relayfs it seemed to be reasonably fast (per cpu buffers; not much locking,
over head per call roughtly like putchar()),
but that might have regressed.

No regression has occurred. According the relay documentation if you use global bufferers you must use locking. If you don't want to use locking use per-cpu bufferers.

Your example module with its lock definitely looks very slow and I don't approve of it.

If you don't approve of the locking then use per-cpu bufferers. The example will do ether.

The example shows a way to create an ASCII data layer.

ASCII layers don't make much sense imho -- these should just use printk.

So the only way I should pass ASCII to user space is using printk? I don't understand that. Again nothing in trace limits you to ASCII data.

Fast dedicated binary log channels make sense though; but you don't
seem really to be very concentrated on that.

I impose no restriction on what type of data you can pass over trace's fast dedicated channels.

True, to make trace "fast" you need a data layer that can handle the requirements of per-cpu buffers. However there are still advantages of trace over printk even when using global bufferers: selectable bufferer sizes,

printk has selectable buffer sizes too.

   "Long term we probably want more complex tracing based on lttng,
    but I'm a big fan of starting out simple and doing incremental

It's just that relayfs + another not simple layer are definitely not simple.

For a simple logger I'm thinking more like something like SGI's old
ktrace module (which undoubtedly many other people have recreated many
times for specific debugging scenarios)

But that all only makes sense if the overhead is really kept low
and i don't see that in your approach.

Is your complaint with the overhead of setting up a trace channel or the overhead of writing to a trace channel? For the later, trace adds almost no overhead on top of relay.

One advantage of the trace approach is separating control and data layers, therefor trace can support multiple data layers to fit multiple requirements.

I have my ideas on how to develop data layer, others may have their own ideas and I welcome the input.

relayfs was supposed to be that data layer.

I am using the layer definitions described in trace.txt. In this definition relay is a buffering layer.

PS: Systemtap has been criticized for introducing out-of-tree kernel code. A clear direction from the community is to move re-usable code in-tree where it can be maintained. Trace is a move in that direction.

I'm all for that. I believe a simple fast efficient no frills logger would serve systemtap just fine too. But the approach here seems to be more to add all kinds of knobs and whizzles until you end up with something as slow with printk. And since we already have printk another one just doesn't seem to make much sense.

If by knobs you mean the trace controls. The only one that has any effect on the "speed" of tracing is the control to start and stop tracing. And that had been designed to impose the minimal impact possible (one "if" in the tracing path).


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