linux-kernel - Re: [patch 4/4] KVM-trace port to tracepoints

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Date:	Wed, 23 Jul 2008 10:55:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	"Feng(Eric) Liu" <eric.e.liu@...el.com>
Subject: Re: [patch 4/4] KVM-trace port to tracepoints

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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