linux-kernel - RE: ptrace API extensions for BTS

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Date:	Fri, 7 Dec 2007 12:01:28 -0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	"Andi Kleen" <ak@...e.de>
Cc:	<roland@...hat.com>, <markus.t.metzger@...il.com>,
	<linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
	<mingo@...e.hu>, "Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: RE: ptrace API extensions for BTS

>From: Andi Kleen [mailto:ak@...e.de] 
>Sent: Freitag, 7. Dezember 2007 12:18

>> I would like to settle the discussion and find an interface that
>> everybody can agree to, so I can implement that interface and we can
>> move forward with the patch.
>
>The most efficient interface would be zero copy with tracer 
>user process
>supplying memory that is pinned (get_user_pages()) subject to the
>mlock rlimit. Then kernel telling the CPU to directly log into
>that.

That would require users to understand all kinds of BTS formats
and to detect the hardware they are running on in order to interpret
the data.

So far, there are two different formats. But one of them is wasting
an entire word of memory per record. I could imagine that this would
change some day.

Other architectures would likely use an entirely different format.
Users who want to support several architectures would benefit from
a common format for this from-to branch information.

>> Regarding 1, we currently provide scheduling timestamps, 
>which are arch
>
>That's actually broken because you don't log the CPU number.
>sched_clock() without the CPU number associated is meaningless 
>on systems without synchronized, pstate invariant TSC 
>[that is older Intel systems or some larger current systems]

I see.

The intention was not to provide exact timestamps, but rather a
relative order of BTS chunks that would allow debuggers to
show which parts were (actually, "might have been" is the best we
can say) executed in parallel, and which parts were definitely 
executed sequentially.

Without a global time, though, this becomes rather meaningless.

Is there some other metric that would allow me to order BTS 
chunks for different threads?

>> Additional architectures may want to (re)use and extend the x86 bts
>> record, or they may want to invent their own format. In the 
>former case,
>
>I think that's actually not a good goal. If the code is so complicated
>that it makes sense sharing then you did something wrong :)

Agreed;-)

Users would benefit if they wanted to support multiple architectures.
They would need to invent such a more general interface; or duplicate 
code, which is never a good thing.

regards,
markus.
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