Date:   Wed, 23 May 2018 10:53:55 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Dave Young <dyoung@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Neil Horman <nhorman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vivek Goyal <vgoyal@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Anton Vorontsov <avorontsov@...mvista.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>, dzickus@...hat.com,
        bhe@...hat.com
Subject: Re: [PATCH] kdump: add default crashkernel reserve kernel config options

Dave Young <dyoung@...hat.com> writes:

> [snip]
>
>> >  
>> > +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 :(
>
> Andrew, I tuned it a bit, removed the check of sz >= system_ram, so if
> the size is too large and kernel can not find enough memory it will
> still fail in latter code.
>
> Is below version looks clearer?

What is the advantage of providing this in a kconfig option rather
than on the kernel command line as we can now?

Eric

> ---
>
> 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.
>
> Signed-off-by: Dave Young <dyoung@...hat.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.
>
>  arch/Kconfig        |   17 +++++++++++++++++
>  kernel/crash_core.c |   19 ++++++++++++++++++-
>  2 files changed, 35 insertions(+), 1 deletion(-)
>
> --- linux-x86.orig/arch/Kconfig
> +++ linux-x86/arch/Kconfig
> @@ -10,6 +10,23 @@ config KEXEC_CORE
>  	select CRASH_CORE
>  	bool
>  
> +config CRASHKERNEL_DEFAULT_THRESHOLD_MB
> +	int "System memory size threshold for using CRASHKERNEL_DEFAULT_MB"
> +	depends on CRASH_CORE
> +	default 0
> +	help
> +	  CRASHKERNEL_DEFAULT_MB will be reserved for kdump if the system
> +	  memory is above or equal to CRASHKERNEL_DEFAULT_THRESHOLD_MB MB.
> +	  It is only effective in case no crashkernel=X parameter is used.
> +
> +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
>  
> --- linux-x86.orig/kernel/crash_core.c
> +++ linux-x86/kernel/crash_core.c
> @@ -143,6 +143,21 @@ 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 system_ram_mb = system_ram >> 20;
> +
> +	if (system_ram_mb < CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB) {
> +		pr_debug("crashkernel: system memory size is lower than %d\n",
> +			 CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB);
> +		return -EINVAL;
> +	}
> +	*size = (unsigned long long)CONFIG_CRASHKERNEL_DEFAULT_MB << 20;
> +
> +	return 0;
> +}
> +
>  #define SUFFIX_HIGH 0
>  #define SUFFIX_LOW  1
>  #define SUFFIX_NULL 2
> @@ -240,8 +255,10 @@ static int __init __parse_crashkernel(ch
>  	*crash_size = 0;
>  	*crash_base = 0;
>  
> -	ck_cmdline = get_last_crashkernel(cmdline, name, suffix);
> +	if (!strstr(cmdline, "crashkernel="))
> +		return get_crashkernel_default(system_ram, crash_size);
>  
> +	ck_cmdline = get_last_crashkernel(cmdline, name, suffix);
>  	if (!ck_cmdline)
>  		return -EINVAL;
>  

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