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Subject: Re: [PATCHv11 2.6.36-rc2-tip 3/15]  3: uprobes: Slot allocation
 for Execution out of line(XOL)
From: Peter Zijlstra <peterz@infradead.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
        Randy Dunlap <rdunlap@xenotime.net>,
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        Andrew Morton <akpm@linux-foundation.org>,
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Date: Thu, 02 Sep 2010 10:23:32 +0200
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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.