netdev - Re: [PATCH 19/30] panic: Add the panic hypervisor notifier list

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Date:   Wed, 18 May 2022 10:24:39 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Evan Green <evgreen@...omium.org>, David Gow <davidgow@...gle.com>,
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Subject: Re: [PATCH 19/30] panic: Add the panic hypervisor notifier list

On 18/05/2022 04:33, Petr Mladek wrote:
> [...]
> Anyway, I would distinguish it the following way.
> 
>   + If the notifier is preserving kernel log then it should be ideally
>     treated as kmsg_dump().
> 
>   + It the notifier is saving another debugging data then it better
>     fits into the "hypervisor" notifier list.
> 
>

Definitely, I agree - it's logical, since we want more info in the logs,
and happens some notifiers running in the informational list do that,
like ftrace_on_oops for example.


> Regarding the reliability. From my POV, any panic notifier enabled
> in a generic kernel should be reliable with more than 99,9%.
> Otherwise, they should not be in the notifier list at all.
> 
> An exception would be a platform-specific notifier that is
> called only on some specific platform and developers maintaining
> this platform agree on this.
> 
> The value "99,9%" is arbitrary. I am not sure if it is realistic
> even in the other code, for example, console_flush_on_panic()
> or emergency_restart(). I just want to point out that the border
> should be rather high. Otherwise we would back in the situation
> where people would want to disable particular notifiers.
> 

Totally agree, these percentages are just an example, 50% is ridiculous
low reliability in my example heheh

But some notifiers deep dive in abstraction layers (like regmap or GPIO
stuff) and it's hard to determine the probability of a lock issue (take
a spinlock already taken inside regmap code and live-lock forever, for
example). These are better to run, if possible, later than kdump or even
info list.

Thanks again for the good analysis Petr!
Cheers,


Guilherme