Date: Wed, 10 Oct 2007 16:53:53 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Darren Pilgrim <freebsd@bitfreak.org> Subject: Re: Non-sequential AP starts [Was: Re: 8-core server] Message-ID: <200710101653.54019.jhb@freebsd.org> In-Reply-To: <46FDD3AB.3050305@bitfreak.org> References: <b41c75520709280103ied4e99dvfd33884185792a91@mail.gmail.com> <20070928184659.GB57646@cicely12.cicely.de> <46FDD3AB.3050305@bitfreak.org>
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On Saturday 29 September 2007 12:25:15 am Darren Pilgrim wrote: > Bernd Walter wrote: > > On Fri, Sep 28, 2007 at 07:56:49AM -0700, Darren Pilgrim wrote: > >> Jeremy Chadwick wrote: > >>> I think what you're not noticing is that the cores are being launched in > >>> non-sequential order. 1, 2, 3, 4, 7, 5, 6. This isn't a problem. > >> Why they wouldn't be launched sequentially? > > > > First SMP rule: > > Don't expect a specific execution order from things running parallel. > > > > I don't know the code, but would assume that they are started > > sequentially and each core prints it's own line, so they get > > disordered. > > This question bugged me long enough for me to go read some source to > find the answer, but I ended up with more questions. I guess that's > what I get for reading boot code. :) I know it's basically just a race > condition, but where does the race actually occur? In each core's > execution of startup instructions leading up to the printf() in > init_secondary() or in the assignment of the value returned by > PCPU_GET(cpudid)? Where is the data behind PCPU_GET() anyway? I > couldn't find it. Is AP launch order random or are there CPU > characteristics that result in reproducible non-sequential ordering? The lauch is random. The boot processor sets a flag to tell the APs (which are already running, but spinnin in a loop) "ok, go run". The APs then compete on a spin lock to serialize their final initialization, and they "launch" in the order they acquire the spin lock, which is quite indeterminate. -- John Baldwin
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