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SubjectRe: [PATCH] kdump: add default crashkernel reserve kernel config options
On Mon, 21 May 2018 10:53:37 +0800 Dave Young <dyoung@redhat.com> wrote:

> This is a rework of the crashkernel=auto patches back to 2009 although
> I'm not sure if below is the last version of the old effort:
> https://lkml.org/lkml/2009/8/12/61
> https://lwn.net/Articles/345344/
>
> I changed the original design, instead of adding the auto reserve logic
> in code, in this patch just introduce two kernel config options for
> the default crashkernel value in MB and the threshold of system memory
> in MB so that only reserve default when system memory is equal or
> above the threshold.
>
> With the kernel configs distributions can easily change the default
> values so that people do not need to manually set kernel cmdline
> for common use cases and one can still overwrite the default value
> with manual setup or disable it by using crashkernel=0
>
> Signed-off-by: Dave Young <dyoung@redhat.com>
> ---
> Another difference is with original design the crashkernel size scales
> with system memory, according to test, large machine may need more
> memory in kdump kernel because of several factors:
> 1. cpu numbers, because of the percpu memory allocated for cpus.
> (kdump can use nr_cpus=1 to workaround this, but some
> arches do not support nr_cpus=X for example powerpc)
> 2. IO devices, large system can have a lot of io devices, although we
> can try to only add those device drivers we needed, it is still a
> problem because of some built-in drivers, some stacked logical devices
> eg. device mapper devices, acpi etc. Even if only considering the
> meta data for driver model it will still be a big number eg. sysfs
> files etc.
> 3. The minimum memory requirement for some device drivers are big, even
> if some of them have implemented low meory profile. It is usual to see
> 10M memory use for a storage driver.
> 4. user space initramfs size growing. Busybox is not usable if we need
> to add udev support and some complicate storage support. Use dracut
> with systemd, especially networking stuff need more memory.
>
> So probably add another kernel config option to scale the memory size
> eg. CRASHKERNEL_DEFAULT_SCALE_RATIO is also good to have, in RHEL we
> use base_value + system_mem >> (2^14) for x86. I'm still hesatating
> how to describe and add this option. Any suggestions will be appreciated.
>
> ...
>
> --- linux-x86.orig/arch/Kconfig
> +++ linux-x86/arch/Kconfig
> @@ -10,6 +10,22 @@ config KEXEC_CORE
> select CRASH_CORE
> bool
>
> +config CRASHKERNEL_DEFAULT_THRESHOLD_MB
> + int "System memory size threshold for kdump memory default reserving"
> + depends on CRASH_CORE
> + default 0
> + help
> + CRASHKERNEL_DEFAULT_MB is used as default crashkernel value if
> + the system memory size is equal or bigger than the threshold.

"the threshold" is rather vague. Can it be clarified?

In fact I'm really struggling to understand the logic here....


> +config CRASHKERNEL_DEFAULT_MB
> + int "Default crashkernel memory size reserved for kdump"
> + depends on CRASH_CORE
> + default 0
> + help
> + This is used as the default kdump reserved memory size in MB.
> + crashkernel=X kernel cmdline can overwrite this value.
> +
> config HAVE_IMA_KEXEC
> bool
>
> @@ -143,6 +144,24 @@ static int __init parse_crashkernel_simp
> return 0;
> }
>
> +static int __init get_crashkernel_default(unsigned long long system_ram,
> + unsigned long long *size)
> +{
> + unsigned long long sz = CONFIG_CRASHKERNEL_DEFAULT_MB;
> + unsigned long long thres = CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB;
> +
> + thres *= SZ_1M;
> + sz *= SZ_1M;
> +
> + if (sz >= system_ram || system_ram < thres) {
> + pr_debug("crashkernel default size can not be used.\n");
> + return -EINVAL;

In other words,

if (system_ram <= CONFIG_CRASHKERNEL_DEFAULT_MB ||
system_ram < CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB)
fail;

yes?

How come? What's happening here? Perhaps a (good) explanatory comment
is needed. And clearer Kconfig text.

All confused :(
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