linux-kernel - Re: [PATCHv11 2.6.36-rc2-tip 3/15] 3: uprobes: Slot allocation for Execution out of line(XOL)

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Message-ID: <1283415812.2059.1825.camel@laptop>
Date:	Thu, 02 Sep 2010 10:23:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Randy Dunlap <rdunlap@...otime.net>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Mark Wielaard <mjw@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naren A Devaiah <naren.devaiah@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv11 2.6.36-rc2-tip 3/15]  3: uprobes: Slot allocation
 for Execution out of line(XOL)

On Wed, 2010-08-25 at 19:11 +0530, Srikar Dronamraju wrote:
> Current slot allocation mechanism:
> 1. Allocate one dedicated slot per user breakpoint. Each slot is big
> enuf to accomodate the biggest instruction for that architecture. (16
> bytes for x86).
> 2. We currently allocate only one page for slots. Hence the number of
> slots is limited to active breakpoint hits on that process.
> 3. Bitmap to track used slots. 

An alternative method would be to have 1 slot per cpu, and manage the
slot content using preemption notifiers. That gives you a fixed number
of slots and an unlimited number of probe points.

If the preemption happens to be a migration you need to rewrite the
userspace IP to point to the new slot -- if indeed the task was inside
one when it got preempted -- but that all should be doable.


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