Re: Problem with simple scheduler

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 02/01/14 10:37, Manohar Vanga wrote:
> Hi Andrew,
>
> Thanks for the quick reply! My comments are inline:
>  
>
>     Using printk()s in the code is going to skew the timing terribly.
>
>
> Would this affect the boot process of dom0 in any way? Other than
> that, it doesn't seem like it would cause any problems right?

Not directly, but it means the symptoms are likely to change simply by
looking for them.

>  
>
>     A serial console and the 'q' debug key is probably a good start,
>     to see some vcpu state.
>
>
> I'll give this a try but the problem is that I'm not entirely sure
> what I'm looking for. As I mentioned, the vcpu of dom0 goes idle after
> the crash-point.

The pause_count and pause_flags are what you are looking for to start with.

>  
>
>     Beyond that, writing a custom keyhandler to dump all of the xfair
>     state is probably the next thing to try.
>
>
> Hmm any hints on what I should be looking for? It's clear to me that
> the vcpu goes idle after that point where dom0 stops booting. Is there
> anything else I need to keep an eye out for? I'll also look into
> whether the skewed timing can possibly lead to a broken dom0 boot.

You have cause and effect mixed up.  (Presumably) the scheduler makes a
mistake and fails to reschedule dom0's vcpu, at which point dom0 makes
no further progress booting.

You will need to work out why the scheduler is not rescheduling the
vcpu, which involves knowing the complete state of the scheduler, and
the vcpu scheduling information.

>
> Also, in case you glanced at the code, do you happen to see any
> glaringly obvious bugs that you might have encountered? Otherwise I
> guess I'm looking for some odd corner-case behavior.

Writing a scheduler is probably the single most complicated task
available.  It is absolutely full of subtle logic issues which are not
obvious in the slightest.

I have no specific knowledge about schedulers or writing them, so really
cant help you too much with the specifics of your code.

~Andrew

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