Re: [patch 4/4] Port of blktrace to the Linux Kernel Markers.

Mathieu Desnoyers
2007-08-30T18:38:16Z

* Christoph Hellwig (hch@infradead.org) wrote:
> On Mon, Aug 27, 2007 at 12:05:44PM -0400, Mathieu Desnoyers wrote:
> > Here is the first stage of a port of blktrace to the Linux Kernel Markers. The
> > advantage of this port is that it minimizes the impact on the running when
> > blktrace is not active.
> >
> > A few remarks : this patch has the positive effect of removing some code
> > from the block io tracing hot paths, minimizing the i-cache impact in a
> > system where the io tracing is compiled in but inactive.
> >
> > It also moves the blk tracing code from a header (and therefore from the
> > body of the instrumented functions) to a separate C file.
> >
> > There, as soon as one device has to be traced, all devices have to
> > execute the tracing function call when they pass by the instrumentation site.
> > This is slower than the previous inline function which tested the condition
> > quickly.
> >
> > It does not make the code smaller, since I left all the specialized
> > tracing functions for requests, bio, generic, remap, which would go away
> > once a generic infrastructure is in place to serialize the information
> > passed to the marker. This is mostly why I consider it as a step towards the
> > full improvements that could bring the markers.
>
> I like this as it moves the whole tracing code out of line. It would
> be nice if we could make blktrace a module with this, but we'd need
> to change the interface away from an ioctl on the block device for that.
>
> Btw, something that really shows here and what I noticed in my sputrace
> aswell is that there is a lot of boilerplate code due to the varargs
> trace handlers. We really need some way to auto-generate the boilerplate
> for the trace function to avoid coding this up everywhere.

Or we can use a vprintk-like function to parse the format string and
serialize the information into trace buffers. I prefer this latter
option because, overall, it will localize the probes in a few bytes of
functions instead of duplicating the memory and instruction cache
required by multiple serializing functions.

I have the code ready, but I do not want to flood LKML with patches
neither....

Mathieu