Re: [patch 48/91] kernel/crash_core: add crashkernel=auto for vmcore creation

[Baoquan He <bhe@redhat.com>]

Add Kairui to CC since he is taking care of the crashkernel=auto code in
our Distros.

On 05/08/21 at 11:13am, Baoquan He wrote:
> Hi Linus,
> 
> On 05/07/21 at 12:25am, Linus Torvalds wrote:
> > On Thu, May 6, 2021 at 6:04 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > From: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
> > > Subject: kernel/crash_core: add crashkernel=auto for vmcore creation
> > >
> > > This adds crashkernel=auto feature to configure reserved memory for vmcore
> > > creation.  CONFIG_CRASH_AUTO_STR is defined to be set for different kernel
> > > distributions and different archs based on their needs.
> 
> 
> > 
> > Ugh. I didn't realize how nasty this was until after I'd applied this patch.
> > 
> > I'm going to drop this patch, because the Kconfig thing for it is an
> > unmitigated mess. I was confused by the question, and then the help
> > message was actively misleading.
> > 
> > This is wrong for so many reasons:
> > 
> >  - this is a classic case of "you shouldn't ask a user this".
> > 
> >    The question makes no sense to any normal person, it certainly
> > didn't to me. Don't ask questions that don't have sane answers.
> > 
> >  - the config help text is actively misleading, and claims that the
> > option is about how much memory is reserved for a crash kernel
> > 
> >    Not so. It's the default string for when somebody uses "crashkernel=auto"
> 
> Sorry for the confusion, we should have been more careful to reivew and
> add the commit log and kernel config description.
> > 
> >  - this shouldn't be a config option at all, it's clearly a distro
> > setting, and should be on the kernel command line with the other
> > distro settings.
> 
> Don't know kernel config is disliked sometime, will remember it in the
> future and more cautiously to add. 
> 
> Crashkernel=auto exists in our distros for many years, and as David
> mentioned in other thread, we have been trying to adding rashkernel=auto
> support into upstream. We pursue crashkernel=auto being added to upstream
> because:
> 
> 1) Empirical value is given to user by default;
> 
> It was required by customer originally, now has been an important part
> of kdump feature and supported in several main ARCHes. With crashkernel=auto,
> people w/o much knowledge of kdump details can use kdump to debug. Distros
> can provide the suggested values with crashkernel=auto which are got by
> investigation, analysis and tested widely on test environment. 
> 
> 2) Cover corner case/special case;
> 
> In some cases, kernel may need extra memory to handle, kdump kernel is
> not exceptional. E.g when sme/sev enabled, SWIOTLB will be enabled
> necessarily, even in kdump kernel. (Below sme/sev related commits for
> reference). Then extra 64M need be reserved for crashkernel. User
> doesn't need to know this, we already have done it for them.
> 
> commit c7753208a94c ("x86, swiotlb: Add memory encryption support")
> commit aba2d9a6385a ("iommu/amd: Do not disable SWIOTLB if SME is active")
> commit d7b417fa08d1 ("x86/mm: Add DMA support for SEV memory encryption")
> 
> We are eager to push crashkernel=auto to upstream becasue of our
> UPSTREAM FIRST rule. Since it has been in RHEL for many years, each time
> a new RHEL main release anchor a upstream kernel release and is prepared,
> these crashkernel=auto RHEL-only patches need be reviewed inside Redhat,
> then we will be questioned and challenged why they are not in upstream.
> 
> As for how to implement crashkernel=auto, we have tried several ways.
> 
> 1) Add into kernel command line
> 
> The suggested value need be stored in user space if added into kernel
> command line, then added into kernel. This makes the suggested value
> separated from kernel itself. It's not what we expect to see. Because
> the suggested crashkernel value is strongly related to distros release.
> We could adjust the value between sub-releases of kernel because of
> of kernel change. Adding them into kernel command line make us lose the
> track of them in kernel.
> 
> 2) Add a weak generic function and several arch dependent functions
> 3) Hardcode values in __parse_crashkernel()
> 
> Method 2) is taken in our RHEL7, 3) is used in RHEL8, RHEL-only patches
> add them. If we try to push them into upstram, any later value
> adjustment need a upstream patch posting. Otherwise, RHEL-only patch
> need be introduced again, Redhat internal reviewer will challenge us
> again. (Put the value hard coding pieces at bottom for reference).
> 
> 4) Add kernel config to add default value
> 
> It's done in this patch. With the kernel config CRASH_AUTO_STR, Distros can
> add default value, and adjust it anytime in the future w/o bothering
> upstream. If crashkernel=auto is specified, only below 3 LOC added, to
> go to parse the CONFIG_CRASH_AUTO_STR directly.
> 
> @@ -250,6 +251,12 @@ static int __init __parse_crashkernel(ch
>         if (suffix)
>                 return parse_crashkernel_suffix(ck_cmdline, crash_size,
>                                 suffix);
> +#ifdef CONFIG_CRASH_AUTO_STR
> +       if (strncmp(ck_cmdline, "auto", 4) == 0) {
> +               ck_cmdline = CONFIG_CRASH_AUTO_STR;
> +               pr_info("Using crashkernel=auto, the size chosen is a best effort estimation.\n");
> +       }
> +#endif
> 
> 
> Before this, we don't know Saeed Mirzamohammadi, the patch author. He
> could experience the same torture. We were wild with joy when noticing
> his patch. We were planning to launch new round of post to add
> crashkernel=auto, kernel config is our final option too. We could be too
> happy to forget polishing the commit log.
> 
> Not sure if I make myself clear. Basically, we expect crashkernel=auto
> to be added in upstream kernel. About how to implement it in kernel, we would
> like to hear upstream people's suggestion.
> 
> Thanks
> Baoquan
> 
> 
> Hard code crashkernel=auto values in __parse_crashkernel()
> ===========================================================
> static int __init __parse_crashkernel(char *cmdline,
>                              unsigned long long system_ram,
>                              unsigned long long *crash_size,
>                              unsigned long long *crash_base,
>                              const char *name,
>                              const char *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");
>         }
> 
> ......
> }
> ==================================================================
> 
> 
> 
>