Messages in this thread |  | | Date | Tue, 17 Jun 2008 00:20:28 +0100 (BST) | From | "Maciej W. Rozycki" <> | Subject | Re: nmi_watchdog suspicious |
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On Mon, 16 Jun 2008, Cyrill Gorcunov wrote:
> Maciej, I think nmi_watchdog could (and probably should) be defined as > unsigned. Here my points of why (fix me please if I'm wrong): > > - if we remain it as unsigned we could simplify setup_nmi_watchdog() to > just check for 'if (nmi >= NMI_INVALID)'
This is run once only at the boot if at all -- just to verify the range is correct. Other places are executed multiple times during normal operation and it is them you should optimise for.
> - current code does check for NMI_NONE _and_ NMI_DISABLED at once in most > cases (only the case it dont is - proc_nmi_enabled() wich could be simplified too)
Please note the intent is NMI_DISABLED is a bootstrap default to tell the platform the user has not specified any override. With the 32-bit platform it used to be promoted automatically to NMI_IO_APIC or NMI_LOCAL_APIC as appropriate, but it was removed because of stability problems with many systems. It looks it wasn't done in a particularly fortunate way -- the new promotion should be to NMI_NONE, but instead it was removed altogether.
Preferably the initialization to NMI_NONE should be done as soon as it has been determined there was no "nmi_watchdog=" option specified, but in practice I think it can simply be done at the beginning of trap_init(), before the gate descriptor has been set up for the NMI (after which point the NMI handler can be reached). This way no piece of code other than setup_nmi_watchdog() would have to care about negative values of nmi_watchdog.
> - the only affected of such sign/unsign contention I found is > touch_nmi_watchdog() for which I suggested the patch (already in Ingo's tip tree) > http://lkml.org/lkml/2008/6/12/200 > So there could be some 'useless counters resetting' but it could happen for > quite short time while APIC in initialization phase.
This is a sloppy coding practice which has led us to the current situation with the APIC code -- there should be no "useless code execution" unless absolutely unavoidable. I'd feel more comfortable if there was a separate variable like nmi_watchdog_active checked in the handler instead of nmi_watchdog that would only be set once the watchdog has actually been activated.
The whole idea of touch_nmi_watchdog() itself is rather unfortunate too, but that's apparently not an easy problem to solve.
Maciej
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