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

[Christoph Hellwig <hch@infradead.org>]

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.
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