linux-kernel - Re: [PATCH] tick: add check for the existence of broadcast clock event device

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Message-ID: <alpine.LFD.2.00.0906080857260.3419@localhost.localdomain>
Date:	Mon, 8 Jun 2009 09:00:14 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Feng Tang <feng.tang@...el.com>
cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"Pan, Jacob jun" <jacob.jun.pan@...el.com>
Subject: Re: [PATCH] tick: add check for the existence of broadcast clock
 event device

On Mon, 8 Jun 2009, Feng Tang wrote:
> On Mon, 8 Jun 2009 14:33:14 +0800 Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Mon, 8 Jun 2009, Feng Tang wrote:
> > > Our apbt driver is pretty similar with HPET's, including its cpu
> > > hotplug notifier. But our platform only has 2 available apbt to
> > > use, otherwise we will configure it just like HPET, using one timer
> > > as bc and others for per-cpu ones, then it won't hit this case
> > > 
> > > There are 2 situations, one is for the normal boot, apbt0 will be
> > > inited first and registered to OS as cpu0's timer, then tsc/lapic
> > > is calculated based on it, and apbt1 is registered later in a
> > > fs_initcall() (just like hpet.c does) after basic kernel core is
> > > up. so the sequence is: apbt0 --> lapic0 --> lapic1 --> apbt1
> > 
> > Hmm, I do not like that at all. That explicitely relies on CPU0 doing
> > some work which will kick CPU1. That's fragile as hell. 
>
> I understand the concern. apbt0 is inited in a very early boot phase when
> the cpu1 is not up yet, and os don't even know wether there is a cpu1, that's
> why we registered apbt1 in fs_initcall(). If we explicitly setup apbt1 when
> OS brings up cpu1, it is a little brutal and not generic as only our platform
> has apbt, and I guess cpu hotplug maintainer won't like it :p

Why is that a problem ? You already have a special case for apbt0 in
the early setup code. So where is the problem when you have an apbt1
init call on CPU1 _before_ the local APIC is initialized on CPU1.

That's definitely saner than relying on magic IPI wakeups.

Thanks,

	tglx
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