Subject: Re: [patch 4/4] KVM-trace port to tracepoints
From: Peter Zijlstra <peterz@infradead.org>
To: Avi Kivity <avi@qumranet.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       akpm@linux-foundation.org, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
       "Feng(Eric) Liu" <eric.e.liu@intel.com>
In-Reply-To: <4886E6FD.4090200@qumranet.com>
References: <20080717155724.897537670@polymtl.ca>
	 <20080717160003.359557938@polymtl.ca> <487F7800.4010502@siemens.com>
	 <20080717172853.GB29855@Krystal> <488604F8.1040008@siemens.com>
	 <48862B01.7070907@qumranet.com> <1216799346.7257.125.camel@twins>
	 <4886E6FD.4090200@qumranet.com>
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Date: Wed, 23 Jul 2008 10:55:03 +0200
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On Wed, 2008-07-23 at 11:08 +0300, Avi Kivity wrote:
> Peter Zijlstra wrote:
> > On Tue, 2008-07-22 at 21:46 +0300, Avi Kivity wrote:
> >   
> >> Jan Kiszka wrote:
> >>     
> >>> That's true - as long as you don't have to add/remove/modify
> >>> tracepoints. I had to do this job in the past (not for KVM). Having 1
> >>> spot in 1 file (based on generic probes) would be handier in that case
> >>> than 5 spots in 3 files. But if the KVM tracepoints are considered
> >>> stable in their number and structure, that shouldn't be an issue here.
> >>>
> >>>   
> >>>       
> >> Tracepoints aren't stable; they are artefacts of the implementation.
> >>     
> >
> > Which IMHO makes it unsuitable for trace_mark() as that will be exported
> > to user-space, and every time you change your tracepoints you'll change
> > user visible things - not nice.
> >   
> 
> They are used for debugging (mostly performance related), so changes are 
> expected.
> 
> What uses of trace_mark() depend on a stable interface? blktrace?

There are currently no trace_mark() sites in the kernel that I'm aware
of (except for the scheduler :-/, and those should be converted to
tracepoints ASAP).

Andrew raised the whole point about trace_mark() generating an
user-visible interface and thus it should be stable, and I agree with
that.

What that means is that trace_mark() can only be used for really stable
points.

This in turn means we might as well use trace points.

Which allows for the conclusion that trace_mark() is not needed and
could be removed from the kernel.

However - it might be handy for ad-hoc debugging purposes that never see
the light of day (linus' git tree in this case). So on those grounds one
could argue against removing trace_mark.


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