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Date:   Mon, 21 May 2018 12:02:15 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Dave Young <dyoung@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Neil Horman <nhorman@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        Tony Luck <tony.luck@intel.com>,
        Anton Vorontsov <avorontsov@ru.mvista.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Hari Bathini <hbathini@linux.vnet.ibm.com>, dzickus@redhat.com,
        bhe@redhat.com
Subject: Re: [PATCH] kdump: add default crashkernel reserve kernel config
 options
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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 :(