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

[Philipp Rudo <prudo@redhat.com>]

Hi Petr,

On Fri, 4 Feb 2022 06:34:19 +0100
Petr Tesařík <ptesarik@suse.cz> wrote:

> Hi Philipp,
> 
> Dne 31. 01. 22 v 11:33 Philipp Rudo napsal(a):
> > Hi,
> > 
> > On Fri, 28 Jan 2022 11:31:49 +0100
> > Petr Tesařík <ptesarik@suse.cz> wrote:
> >   
> >> 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.  
> > 
> > Even this approach doesn't work in every situation. For example it
> > requires that the system has at least twice the RAM it requires to
> > safely boot. That's not always given for e.g minimalistic VMs or
> > embedded systems.  
> 
> If you reserve more RAM for the panic kernel than for running your 
> actual workload, then you definitely have very special needs, and you 
> should not expect that everything works out of the box.

That was basically the point I was trying to make. There is always a
scenario with special needs so that is is basically impossible to find
that one solution that works for everybody.

> > Furthermore the memory requirement can also change during runtime due
> > to, e.g. workload spikes, device hot plug, moving the dump target from
> > an un-encrypted to an encrypted disk, etc.. So even when your user-space
> > program can exactly calculate the memory requirement at the moment
> > it loads kdump it might be too little at the moment the system panics.
> > In order for it to work the user-space would constantly need to monitor
> > how much memory is needed and adjust the requirement. But that would
> > also require to increase the reservation during runtime which would be
> > extremely expensive (if possible at all).
> > 
> > All in all I support Petr that time travel is the only proper solution
> > for implementing crashkernel=auto. But once we have time travel I
> > would prefer to use the gained knowledge to fix the bug that triggered
> > the panic rather than calculating the memory requirement for kdump.  
> 
> Yeah, long live patching! :-)
> 
> >> The alternative approach does not need any changes to the kernel, except
> >> maybe adding something like "crashkernel=max".  
> > 
> > A slightly different approach is for the user-space tool to simply set
> > the crashkernel= parameter on the kernel commandline for the next boot.
> > This also works for memory restrained systems. Needs a reboot though...  
> 
> The downside is that if you remove some memory while your system is off, 
> then a reservation calculate for the previous RAM size may no longer be 
> possible on the next boot, and the kernel will boot up without any 
> reservation. That's where "crashkernel=max" would come in handy. Let me 
> send a patch and see the discussion.

True, in that situation our approach will fail. I'm looking forward to
see your patch.

Thanks
Philipp

> >>> 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@oracle.com/  
> > 
> > The patch was ultimately rejected by Linus
> > 
> > https://lore.kernel.org/linux-mm/20210507010432.IN24PudKT%25akpm@linux-foundation.org/
> > 
> > Thanks
> > Philipp
> >   
> >>>      
> >>>>     
> >>>>> [...]
> >>>>> 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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