Re: Non-sequential AP starts [Was: Re: 8-core server]

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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