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Subject: Re: [PATCH 24/30] panic: Refactor the panic path
Thread-Topic: [PATCH 24/30] panic: Refactor the panic path
Thread-Index: AQHYWooYRc5K2GmPwUyqsvUuWmK42q0WuNLD
Date:   Mon, 9 May 2022 15:16:08 +0000
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References: <20220427224924.592546-1-gpiccoli@igalia.com>
 <20220427224924.592546-25-gpiccoli@igalia.com>
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Sorry for the delayed response. Unfortunately, I had 10 days holidays=0A=
until yesterday...=0A=
=0A=
>  .../admin-guide/kernel-parameters.txt         |  42 ++-=0A=
>  include/linux/panic_notifier.h                |   1 +=0A=
>  kernel/kexec_core.c                           |   8 +-=0A=
>  kernel/panic.c                                | 292 +++++++++++++-----=
=0A=
>  .../selftests/pstore/pstore_crash_test        |   5 +-=0A=
>  5 files changed, 252 insertions(+), 96 deletions(-)=0A=
> =0A=
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentat=
ion/admin-guide/kernel-parameters.txt=0A=
> index 3f1cc5e317ed..8d3524060ce3 100644=0A=
> --- a/Documentation/admin-guide/kernel-parameters.txt=0A=
> +++ b/Documentation/admin-guide/kernel-parameters.txt=0A=
...snip...=0A=
> @@ -3784,6 +3791,33 @@=0A=
>                         timeout < 0: reboot immediately=0A=
>                         Format: <timeout>=0A=
> =0A=
> +       panic_notifiers_level=3D=0A=
> +                       [KNL] Set the panic notifiers execution order.=0A=
> +                       Format: <unsigned int>=0A=
> +                       We currently have 4 lists of panic notifiers; bas=
ed=0A=
> +                       on the functionality and risk (for panic success)=
 the=0A=
> +                       callbacks are added in a given list. The lists ar=
e:=0A=
> +                       - hypervisor/FW notification list (low risk);=0A=
> +                       - informational list (low/medium risk);=0A=
> +                       - pre_reboot list (higher risk);=0A=
> +                       - post_reboot list (only run late in panic and af=
ter=0A=
> +                       kdump, not configurable for now).=0A=
> +                       This parameter defines the ordering of the first =
3=0A=
> +                       lists with regards to kdump; the levels determine=
=0A=
> +                       which set of notifiers execute before kdump. The=
=0A=
> +                       accepted levels are:=0A=
> +                       0: kdump is the first thing to run, NO list is=0A=
> +                       executed before kdump.=0A=
> +                       1: only the hypervisor list is executed before kd=
ump.=0A=
> +                       2 (default level): the hypervisor list and (*if*=
=0A=
=0A=
Hmmm, why are you trying to change default setting?=0A=
=0A=
Based on the current design of kdump, it's natural to put what the=0A=
handlers for these level 1 and level 2 handlers do in=0A=
machine_crash_shutdown(), as these are necessary by default, right?=0A=
=0A=
Or have you already tried that and figured out it's difficult in some=0A=
reason and reached the current design? If so, why is that difficult?=0A=
Could you point to if there is already such discussion online?=0A=
=0A=
kdump is designed to perform as little things as possible before=0A=
transferring the execution to the 2nd kernel in order to increase=0A=
reliability. Just detour to panic() increases risks of kdump failure=0A=
in the sense of increasing the executed codes in the abnormal=0A=
situation, which is very the note in the explanation of=0A=
crash_kexec_post_notifiers.=0A=
=0A=
Also, the current implementation of crash_kexec_post_notifiers uses=0A=
the panic notifier, but this is not from the technical=0A=
reason. Ideally, it should have been implemented in the context of=0A=
crash_kexec() independently of panic().=0A=
=0A=
That is, it looks to me that, in addition to changing design of panic=0A=
notifier, you are trying to integrate shutdown code of the crash kexec=0A=
and the panic paths. If so, this is a big design change for kdump.=0A=
I'm concerned about increase of reliability. I'd like you to discuss=0A=
them carefully.=0A=
=0A=
Thanks.=0A=
HATAYAMA, Daisuke=0A=
=0A=