linux-kernel - Re: [PATCH v2 1/1] kexec: dump kmessage before machine

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Date:   Thu, 28 Jan 2021 14:00:28 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     jmorris@...ei.org, sashal@...nel.org, tyhicks@...ux.microsoft.com,
        pmladek@...e.com, keescook@...omium.org, anton@...msg.org,
        ccross@...roid.com, tony.luck@...el.com, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] kexec: dump kmessage before machine_kexec

Pavel Tatashin <pasha.tatashin@...een.com> writes:

> kmsg_dump(KMSG_DUMP_SHUTDOWN) is called before
> machine_restart(), machine_halt(), machine_power_off(), the only one that
> is missing is  machine_kexec().
>
> The dmesg output that it contains can be used to study the shutdown
> performance of both kernel and systemd during kexec reboot.
>
> Here is example of dmesg data collected after kexec:

As long was we keep kmsg_dump out of the crash_kexec path where
it completely breaks kexec on panic this seems a reasonable thing to do.
On the ordinary kernel_kexec path everything is expected to be working.

Is kmsg_dump expected to work after all of the device drivers
are shut down?  Otherwise this placement of kmsg_dump is too late.

Eric

> root@...at-cp22:~# cat /sys/fs/pstore/dmesg-ramoops-0 | tail
> ...
> <6>[   70.914592] psci: CPU3 killed (polled 0 ms)
> <5>[   70.915705] CPU4: shutdown
> <6>[   70.916643] psci: CPU4 killed (polled 4 ms)
> <5>[   70.917715] CPU5: shutdown
> <6>[   70.918725] psci: CPU5 killed (polled 0 ms)
> <5>[   70.919704] CPU6: shutdown
> <6>[   70.920726] psci: CPU6 killed (polled 4 ms)
> <5>[   70.921642] CPU7: shutdown
> <6>[   70.922650] psci: CPU7 killed (polled 0 ms)
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> Reviewed-by: Bhupesh Sharma <bhsharma@...hat.com>
> ---
>  kernel/kexec_core.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 4f8efc278aa7..e253c8b59145 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -37,6 +37,7 @@
>  #include <linux/compiler.h>
>  #include <linux/hugetlb.h>
>  #include <linux/objtool.h>
> +#include <linux/kmsg_dump.h>
>  
>  #include <asm/page.h>
>  #include <asm/sections.h>
> @@ -1180,6 +1181,7 @@ int kernel_kexec(void)
>  		machine_shutdown();
>  	}
>  
> +	kmsg_dump(KMSG_DUMP_SHUTDOWN);
>  	machine_kexec(kexec_image);
>  
>  #ifdef CONFIG_KEXEC_JUMP