linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Message-ID: <alpine.LFD.1.10.0809251448340.3265@nehalem.linux-foundation.org>
Date:	Thu, 25 Sep 2008 14:55:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Ingo Molnar <mingo@...e.hu>, Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...igh.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Tom Zanussi <zanussi@...cast.net>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer

On Thu, 25 Sep 2008, Jeremy Fitzhardinge wrote:
>
> That suggests that frequency changes should be recorded at a lower layer
> as well

Yes and no.

The reason I say "and no" is that it's not technically really possible to 
atomically give the exact TSC at which the frequency change took place. We 
just don't have the information, and I doubt we will ever have it.

As such, there is no point in trying to make it a low-level special op, 
because we'd _still_ end up being totally equivalent with just doing as 
regular trace-event, with a regular TSC field, and then just fill the data 
field with the new frequency.

But yes, I do think we'd need to have that as a trace packet type. I 
thought I even said so in my RFC for packet types. Ahh, it was in the 
follow-up:

> I guess I should perhaps have put the TSC frequency in there in that "case 
> 2" thing too. Maybe that should be in "data" (in kHz) and tv_sec/tv_nsec 
> should be in array[0..1], and the time sync packet would be 24 bytes.

but yes, we obviously need the frequency in order to calculate some kind 
of wall-clock time (it doesn't _have_ to be in the same packet type as the 
thing that tries to sync with a real clock, but it makes sense for it to 
be there.

That said, if people think they can do a good job of ns conversion, I'll 
stop arguing. Quite frankly, I think people are wrong about that, and 
quite frankly, I think that anybody who looks even for one second at those 
"alternate" sched_clock() implementations should realize that they aren't 
suitable, but whatever. I'm not writing the code, I can only try to 
convince people to not add the insane call-chains we have now.

			Linus
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