Subject: Re: [RFC PATCH 02/10] fadump: Reserve the memory for firmware assisted dump.

From: Mahesh Jagannath Salgaonkar <[email-masked]>
Date: Tue, 06 Sep 2011 17:29:55 +0530

Hi Anton,

On 08/31/2011 09:41 AM, Anton Blanchard wrote:
> 
> Hi Mahesh,
> 
> Just a few comments.
> 
>> +#define RMR_START	0x0
>> +#define RMR_END		(0x1UL << 28)	/* 256 MB */
> 
> What if the RMO is bigger than 256MB? Should we be using ppc64_rma_size?

The idea was to have a minimum memory threshold that requires for a
kernel to boot successfully. On some Power systems where RMO is 128MB,
it still requires minimum of 256MB for kernel to boot successfully.

I think we can rename above #defines as BOOT_MEM_START and BOOT_MEM_END
respectively and have BOOT_MEM_END defined as below:

#define BOOT_MEM_END 	((ppc64_rma_size < (0x1UL << 28)) ? \
			(0x1UL << 28) : ppc64_rma_size)

What do you think?

> 
>> +#ifdef DEBUG
>> +#define PREFIX		"fadump: "
>> +#define DBG(fmt...)	printk(KERN_ERR PREFIX fmt)
>> +#else
>> +#define DBG(fmt...)
>> +#endif
> 
> We should use the standard debug macros (pr_debug etc).

Sure will do that.

> 
>> +/* Global variable to hold firmware assisted dump configuration info. */
>> +static struct fw_dump fw_dump;
> 
> You can remove this comment, especially because the variable isn't global :)

Agree.

> 
>> +	sections = of_get_flat_dt_prop(node, "ibm,configure-kernel-dump-sizes",
>> +					NULL);
>> +
>> +	if (!sections)
>> +		return 0;
>> +
>> +	for (i = 0; i < FW_DUMP_NUM_SECTIONS; i++) {
>> +		switch (sections[i].dump_section) {
>> +		case FADUMP_CPU_STATE_DATA:
>> +			fw_dump.cpu_state_data_size =
>> sections[i].section_size;
>> +			break;
>> +		case FADUMP_HPTE_REGION:
>> +			fw_dump.hpte_region_size =
>> sections[i].section_size;
>> +			break;
>> +		}
>> +	}
>> +	return 1;
>> +}
> 
> This makes me a bit nervous. We should really get the size of the property
> and use it to iterate through the array. I saw no requirement in the PAPR
> that the array had to be 2 entries long.
> 

Agree. Will make the change.

>> +static inline unsigned long calculate_reserve_size(void)
>> +{
>> +	unsigned long size;
>> +
>> +	/* divide by 20 to get 5% of value */
>> +	size = memblock_end_of_DRAM();
>> +	do_div(size, 20);
>> +
>> +	/* round it down in multiples of 256 */
>> +	size = size & ~0x0FFFFFFFUL;
>> +
>> +	/* Truncate to memory_limit. We don't want to over reserve
>> the memory.*/
>> +	if (memory_limit && size > memory_limit)
>> +		size = memory_limit;
>> +
>> +	return (size > RMR_END ? size : RMR_END);
>> +}
> 
> 5% is pretty aribitrary, that's 400GB on an 8TB box. Also our experience
> with kdump is that 256MB is too small. Is there any reason to scale it
> with memory size? Can we do what kdump does and set it to a single
> value (eg 512MB)?

I have picked up this heuristic from the phyp-assisted dump code. I am
yet to figure out a fool-proof method to calculate the minimum memory
needed for any Power box to successfully boot. Till then, I presume we
can use this heuristic based approach?

While testing these patches on huge power system with 1TB RAM and 896
CPUs, I found that even 512MB is small. Hence setting it to a single
value may not work for all system configuration.

> 
> We could override the default with a boot option, which is similar to
> how kdump specifies the region to reserve.

Agree, will work on the change.

Thanks,
-Mahesh.