linux-kernel - Re: [RFC PATCH 4/6] relay: add relay_reserve

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Date:	Thu, 05 Jul 2007 11:00:35 -0500
From:	Tom Zanussi <zanussi@...ibm.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	linux-kernel@...r.kernel.org, dwilder@...ibm.com,
	HOLZHEU@...ibm.com
Subject: Re: [RFC PATCH 4/6] relay: add relay_reserve_cpu()

On Tue, 2007-07-03 at 23:17 -0400, Mathieu Desnoyers wrote:
> * Tom Zanussi (zanussi@...ibm.com) wrote:
> > This patch adds the ability to explicitly specify the per-cpu buffer
> > to reserve space in.  Needed for early DTI tracing.
> > 
> 
> This should come with the ability to tell relay where the already
> allocated static buffers are. Can you tell me a little more on why the
> standard relay_reserve would not do the job? Is smp_processor_id() not
> ready to output a sensible CPU id yet ?
> 

In the case of early tracing, the DTI code uses a global buffer and
saves the cpuid along with each event.  Later, when the relay channel is
available, the relogging code wants to put each event in the appropriate
per-cpu relay buffer.  The standard relay_reserve() always uses the
current cpuid, rather than the cpuid attached to the event, which is not
what we want.  relay_reserve_cpu() simply allows a cpuid other than the
current one to be used.

Tom

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