Re: [PATCH v3 0/8] [RfC] fix tracing for modules

[Daniel P . Berrangé]

On Fri, Apr 09, 2021 at 03:12:45PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > eg a trace point "dma_map_wait" gets mapped to probes in many
> > .stp files, once per target, because we need to match based on
> > the executable path:
> > 
> >   probe qemu.system.x86_64.dma_map_wait = 
> > process("/usr/libexec/qemu-system-x86_64").mark("dma_map_wait")
> 
> So, that changes with modules, we need the module name now, i.e.
> 
>     probe qemu.system.x86_64.qxl_soft_reset = \
>       
> process("/home/kraxel/qemu-install/lib/qemu/hw-display-qxl.so").mark("qxl_soft_reset")
> 
> We could repeat that in every qemu-system-$arch.stp file.

This would have the surprise the 'qemu.system.x86_64.qxl_soft_reset'
probes will fire even for qemu-system-ppc64 / qemu-system-xxxxx etc
because we've not restricted the scope as the original probe did.

If we can't fix that, then we must use the second option to avoid
the surprise IMHO

> We could also have one stp file per module, with probes like this:
> 
>     probe qemu.modules.qxl_soft_reset = \
>         
> process("/home/kraxel/qemu-install/lib/qemu/hw-display-qxl.so").mark("qxl_soft_reset")
> 
> The later looks like a better fit to me, but has the drawback that the
> tracepoints have different names in modular and non-modular builds ...

Regards,
Daniel
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