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

[Petr Tesařík]

Hi Tiezhu Yang,

On Jan 28, 2022 at 02:20 Tiezhu Yang wrote:
> [...]
> 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.

Well, the point I'm trying to make is that crashkernel=auto cannot be 
implemented. Your code would have to know what happens in the future, 
and AFAIK time travel has not been discovered yet. ;-)

A better approach is to make a very large allocation initially, e.g. 
half of available RAM. The remaining RAM should still be big enough to 
start booting the system. Later, when a kdump user-space service knows 
what it wants to load, it can shrink the reservation by writing a lower 
value into /sys/kernel/kexec_crash_size.

The alternative approach does not need any changes to the kernel, except 
maybe adding something like "crashkernel=max".

Just my two cents,
Petr T

> 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
>
>
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