Messages in this thread | | | Date | 11 Feb 2006 19:04:59 +0100 | Date | Sat, 11 Feb 2006 19:04:59 +0100 | From | Andi Kleen <> | Subject | Re: [PATCH] percpu data: only iterate over possible CPUs |
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On Sat, Feb 11, 2006 at 03:49:29PM +0100, Heiko Carstens wrote: > > > > x86-64 had the same problem, but we now require that you > > > > boot with additional_cpus=... for how many you want. Default is 0 > > > > (used to be half available CPUs but that lead to confusion) > > > > > > So introducing the additional_cpus kernel parameter seems to be the way > > > to go (for XEN probably too). Even though it seems to be a bit odd if the > > > user specifies both maxcpus=... and additional_cpus=... > > > > With additional_cpus you don't need maxcpus. They are added together. > > How does x86_64 manage to get 'additional_cpus' parsed early enough? As far > as I can see this is done when parse_args() in start_kernel() gets called, > but that's after you need the parameter in prefill_possible_map(). > IMHO that should be an early_param and you would need to call > parse_early_param() from setup_arch(). But then again, maybe I got it all > wrong.
Yes, you're right - it's added too late to the map right now. I will fix that. There are no earlyparams unfortunately, except for a big hack in setup.c
> But the more interesting question is: what do you do if the command line > contains both additional_cpus and maxcpus. I was just trying to make some > sense of this, but the result is questionable. > I ended up with a cpu_possible_map that has 'present cpus' + > 'additional_cpus' bits set. And in smp_prepare_cpus I make sure that > cpu_present_map has not more than max_cpus bits set. > > At least that doesn't break the current semantics of the maxcpus parameter. > But we're still wasting memory, since it would make sense that the > cpu_possible_map shouldn't have more than max_cpus bits set.
Yes, maybe it should be a early parameter too. But frankly I see maxcpus more as a debugging hack or workarouno. I don't think it matters much if it's not as efficient as it could be.
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