linux-kernel - Re: [PATCH 1/4] ksym_tracer: Fix to make the tracer work

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Message-ID: <20100110031056.GA15195@nowhere>
Date:	Sun, 10 Jan 2010 04:10:57 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	prasad@...ux.vnet.ibm.com, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] ksym_tracer: Fix to make the tracer work

On Thu, Dec 31, 2009 at 01:54:35PM +0800, Li Zefan wrote:
> K.Prasad wrote:
> > Frederic must have used for_each_possible_cpu() to account for CPUs that
> > are offline at the time of registration, but may eventually turn online.
> > Since register_wide_hw_breakpoint() interface is designed to deliver
> > system-wide breakpoints, the debug registers of a new online CPU will
> > should have the breakpoints populated to comprehensively notify all
> > memory accesses over target address.
> > 
> > I'd rather wait to hear from Frederic to know why
> > perf_event_create_kernel_counter() returns an error when run for an
> > offline cpu and how it can be solved.
> > 
> 
> See the comment in find_get_context() in kernel/perf_event.c:
> 
>                 /*
>                  * We could be clever and allow to attach a event to an
>                  * offline CPU and activate it when the CPU comes up, but
>                  * that's for later.
>                  */
>                 if (!cpu_online(cpu))
>                         return ERR_PTR(-ENODEV);
> 
> So I think we can use for_each_possible_cpu() in the future, but not now.
> 


Ah, right I indeed missed that.

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