linux-kernel - Re: Two 2.6.20-rc5-rt2 issues

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Date:	Tue, 16 Jan 2007 12:56:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Rui Nuno Capela <rncbc@...bc.org>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Two 2.6.20-rc5-rt2 issues

* Rui Nuno Capela <rncbc@...bc.org> wrote:

> First one is about building for UP (CONFIG_SMP not set) on my old P4 
> laptop. As it seems, all my build attempts failed at the final link 
> stage, with undefined references to paravirt_enable. After disabling 
> CONFIG_PARAVIRT I get a similar failure, but this time for a couple 
> kvm* symbols. [...]

ok, i think i have managed to fix both bugs. I have released -rt3, 
please re-check whether it works any better. If it still doesnt then 
please send me the exact .config that fails.

> [...] I could only get a clean build when CONFIG_SMP is set, which is 
> (IMHO) overkill for a machine which is neither HyperThread/SMT enabled 
> nor multi-core. Its plain dead UP and it used to run PREEMPT_RT 
> kernels for a long time now.

btw., latest SMP kernels (2.6.18 and later) "patch themselves back" to 
UP instructions to a high degree if they run on a single CPU - that's 
why for example Fedora uses only an SMP kernel these days. Running a 
genuine UP kernel is still more efficient - but the difference shouldnt 
be /that/ large anymore. (if someone would like to measure it that would 
be interesting to see)

> Second one is already about running SMP, on a Dual Core2 T7200, for 
> which the build goes fine but run-time is haunted by a crippling BUG:

> Call Trace:
>  [<c0102dad>] __switch_to+0xcc/0x176
>  [<c01185c8>] wake_up_process+0x19/0x1b
>  [<c01fe568>] acpi_ec_gpe_handler+0x1f/0x53
>  [<c01ec6c6>] acpi_ev_gpe_dispatch+0x64/0x163
>  [<c01eca06>] acpi_ev_gpe_detect+0x94/0xd7

hm, this is a -rt specific thing that i hoped to have worked around but 
apparently not. The ACPI code uses a waitqueue in its idle routine 
(argh!) which cannot by done sanely on PREEMPT_RT. In -rt3 i've added a 
more conservative (but still ugly and incorrect) hack - could you try 
it, does -rt3 work any better?

	Ingo
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