Re: [RFC PATCH] kdump: Add support for crashkernel=auto

[Tiezhu Yang]

On 01/27/2022 11:53 PM, Petr Tesařík wrote:
> Hi Tiezhu Yang,
>
> I'm afraid the whole concept is broken by design. See below.
>
> Dne 27. 01. 22 v 10:31 Tiezhu Yang napsal(a):
> > Set the reserved memory automatically for the crash kernel based on
> > architecture.
> >
> > Most code of this patch come from:
> > https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-8/-/tree/c8s
>
> And that's the problem, I think. The solution might be good for this
> specific OS, but not for others.

Hi Petr,

Thank you for your reply.

This is a RFC patch, the initial aim of this patch is to discuss what is 
the proper way to support crashkernel=auto.

A moment ago, I find the following patch, it is more flexible, but it is 
not merged into the upstream kernel now.

kernel/crash_core: Add crashkernel=auto for vmcore creation

https://lore.kernel.org/lkml/20210223174153.72802-1-saeed.mirzamohammadi@xxxxxxxxxx/

> > [...]
> > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > index 256cf6d..32c51e2 100644
> > --- a/kernel/crash_core.c
> > +++ b/kernel/crash_core.c
> > @@ -252,6 +252,26 @@ static int __init __parse_crashkernel(char *cmdline,
> >       if (suffix)
> >           return parse_crashkernel_suffix(ck_cmdline, crash_size,
> >                   suffix);
> > +
> > +    if (strncmp(ck_cmdline, "auto", 4) == 0) {
> > +#if defined(CONFIG_X86_64) || defined(CONFIG_S390)
> > +        ck_cmdline = "1G-4G:160M,4G-64G:192M,64G-1T:256M,1T-:512M";
> > +#elif defined(CONFIG_ARM64)
> > +        ck_cmdline = "2G-:448M";
> > +#elif defined(CONFIG_PPC64)
> > +        char *fadump_cmdline;
> > +
> > +        fadump_cmdline = get_last_crashkernel(cmdline, "fadump=", NULL);
> > +        fadump_cmdline = fadump_cmdline ?
> > +                fadump_cmdline + strlen("fadump=") : NULL;
> > +        if (!fadump_cmdline || (strncmp(fadump_cmdline, "off", 3) == 0))
> > +            ck_cmdline =
> > "2G-4G:384M,4G-16G:512M,16G-64G:1G,64G-128G:2G,128G-:4G";
> > +        else
> > +            ck_cmdline =
> > "4G-16G:768M,16G-64G:1G,64G-128G:2G,128G-1T:4G,1T-2T:6G,2T-4T:12G,4T-8T:20G,8T-16T:36G,16T-32T:64G,32T-64T:128G,64T-:180G";
> >
> > +#endif
> > +        pr_info("Using crashkernel=auto, the size chosen is a best
> > effort estimation.\n");
> > +    }
> > +
>
> How did you even arrive at the above numbers?

Memory requirements for kdump:

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/managing_monitoring_and_updating_the_kernel/supported-kdump-configurations-and-targets_managing-monitoring-and-updating-the-kernel#memory-requirements-for-kdump_supported-kdump-configurations-and-targets

I've done some research on
> this topic recently (ie. during the last 7 years or so). My x86_64
> system with 8G RAM running openSUSE Leap 15.3 seems needs 188M for
> saving to the local disk, and 203M to save over the network (using
> SFTP). My PPC64 LPAR with 16G RAM running latest Beta of SLES 15 SP4
> needs 587M, i.e. with the above numbers it may run out of memory while
> saving the dump.
>
> Since this is not the first time, I'm trying to explain things, I've
> written a blog post now:
>
> https://sigillatum.tesarici.cz/2022-01-27-whats-wrong-with-crashkernel-auto.html

Thank you, this is useful.

Thanks,
Tiezhu

> HTH
> Petr Tesarik