linux-kernel - Re: x86, ptrace: support for branch trace store(BTS)

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Date:	Thu, 13 Dec 2007 14:08:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	ak@...e.de, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, markus.t.metzger@...il.com,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>, roland@...hat.com,
	akpm@...ux-foundation.org, mtk.manpages@...il.com,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: x86, ptrace: support for branch trace store(BTS)


* Metzger, Markus T <markus.t.metzger@...el.com> wrote:

> The ptrace API would allow the user to:
> - define (and query) the overflow mechanism 
>   (wrap-around or event)
> - define (and query) the size of the buffer within certain limits
>   (we could either give an error or cut off)
> - define (and query) events to be monitored
>   (last branch trace, scheduling timestamps)
> - get a single BTS record
> - query the number of BTS records
>   (to find out how big your drain buffer needs to be; it may be bigger
> than you requested)
> - drain all BTS records (copy, then clear)
> - clear all BTS records
> 
> Draining would require the user to allocate a buffer to hold the data, 
> which might not be feasible when he is near his memory limit. He could 
> fall back to looping over the single-entry get. It is questionable, 
> how useful the drain ptrace command would actually be; we might want 
> to replace it with a get range command.
> 
> Are you OK with this?

this sounds a lot more flexible to me. Please, once it looks good to all 
of us also extend LTP's ptrace bits with unit tests for these API 
additions. (Cc: such LTP bits to subrata@...ux.vnet.ibm.com)

	Ingo
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