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Re: A blktrace tapset, or 101 things you can do with blktrace and systemtap

On Tue, 2007-03-06 at 10:53 -0500, Frank Ch. Eigler wrote:
> Hi -
> > > Is there some way for the systemtap script to perform the tracing
> > > activation done by the btrace tool?
> > 
> > AFAIK, there's no API in blktrace that would allow the runtime to easily
> > toggle tracing on/off from the kernel side. [...]
> One would have to simulate the BLKTRACE* ioctls.  Since a systemtap
> script would absorb the tracing data during its creation, blktrace
> proper could be told to keep only a negligible buffer for itself.

Yeah, it looks like that's probably the case.

It would be possible to bypass most of that by just arranging to have
__blk_add_trace() always called if some flag is set, regardless of
whether actual blktrace tracing is going on or not (if not, it would
just return).  That would be sufficient for getting the probe fired and
the data into systemtap, but as mentioned, it would add a couple of
tests to the logging path, so isn't really an option, AFAICS.


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