Re: [RFC] - Mapping ACPI tables as CACHED

[Jack Steiner <steiner@sgi.com>]

On Thu, Jul 22, 2010 at 10:22:20AM -0500, Jack Steiner wrote:
> I'd like feedback on the following performance problem &
> suggestions for a proper solution.

I did some more digging & found something I should have seen earlier.
On EFI-enabled systems (like UV), the ACPI tables are already mapped as
WB memory. This is done in the EFI function efi_enter_virtual_mode().

The E820_ACPI & E820_NVS regions will be mapped as WB memory if the BIOS
sets the WB attribute in the EFI memmap entry for the chunk of memory.

The ACPI code in acpi_os_map_memory() is not currently aware of the EFI mapping
& currently maps the memory as UC. This seems like a bug.

In order to prevent attribute aliasing, I think the ACPI mapping needs to
be consistent with the EFI mapping.  Otherwise, we will have the OS
referencing the memory as UC at the same time BIOS is referencing it as WB.


> 
> 
> Large SGI UV systems (3072p, 5TB) take a long time to boot. A significant
> part of the boot time is scanning ACPI tables. ACPI tables on UV systems
> are located in RAM memory that is physically attached to node 0.
> 
> User programs (ex., acpidump) read the ACPI tables by mapping them thru
> /dev/mem.  Although mmap tries to map the tables as CACHED, there are
> existing kernel UNCACHED mapping that conflict and the tables end up as
> being mapped UNCACHED.  (See the call to track_pfn_vma_new() in
> remap_pfn_range()).
> 
> Much of the access is to small fields (bytes (checksums), shorts, etc).
> Late in boot, there is significant scanning of the ACPI tables that take
> place from nodes other than zero. Since the tables are not cached, each
> reference accesses physical memory that is attached to remote nodes. These
> memory requests must cross the numalink interconnect which adds several
> hundred nsec to each access. This slows the boot process.  Access from
> node 0, although faster, is still very slow.
> 
> 
> 
> The following experimental patch changes the kernel mapping for ACPI tables
> to CACHED. This eliminates the page attibute conflict & allows users to map
> the tables CACHEABLE. This significantly speeds up boot:
> 
> 	38 minutes without the patch
> 	27 minutes with the patch
> 		~30% improvement
> 
> Time to run ACPIDUMP on a large system:
> 	527 seconds without the patch
> 	  8 seconds with the patch
> 
> 
> I don't know if the patch in it's current form is the correct solution. I'm
> interested in feedback on how this should be solved. I expect there
> are issues on other platforms so for now, the patch uses x86_platform_ops
> to change mappings only on UV platforms (I'm paranoid :-).
> 
> I also need to experiment with early_ioremap'ing of the ACPI tables. I suspect
> this is also mapped UNCACHED. There may be additional improvements if this
> could be mapped CACHED. However, the potential performance gain is much
> less since these references all occur from node 0.
> 
> 
> 
> Signed-off-by: Jack Steiner <steiner@sgi.com>
> 
> 
> ---
>  arch/x86/include/asm/x86_init.h    |    2 ++
>  arch/x86/kernel/apic/x2apic_uv_x.c |    6 ++++++
>  arch/x86/kernel/x86_init.c         |    3 +++
>  drivers/acpi/osl.c                 |   12 +++++++++---
>  4 files changed, 20 insertions(+), 3 deletions(-)
> 
> Index: linux/arch/x86/include/asm/x86_init.h
> ===================================================================
> --- linux.orig/arch/x86/include/asm/x86_init.h	2010-07-21 16:53:30.226241589 -0500
> +++ linux/arch/x86/include/asm/x86_init.h	2010-07-21 16:57:46.614872338 -0500
> @@ -113,6 +113,7 @@ struct x86_cpuinit_ops {
>  
>  /**
>   * struct x86_platform_ops - platform specific runtime functions
> + * @is_wb_acpi_tables		E820 ACPI table are in WB memory
>   * @is_untracked_pat_range	exclude from PAT logic
>   * @calibrate_tsc:		calibrate TSC
>   * @get_wallclock:		get time from HW clock like RTC etc.
> @@ -120,6 +121,7 @@ struct x86_cpuinit_ops {
>   * @nmi_init			enable NMI on cpus
>   */
>  struct x86_platform_ops {
> +	int (*is_wb_acpi_tables)(void);
>  	int (*is_untracked_pat_range)(u64 start, u64 end);
>  	unsigned long (*calibrate_tsc)(void);
>  	unsigned long (*get_wallclock)(void);
> Index: linux/arch/x86/kernel/apic/x2apic_uv_x.c
> ===================================================================
> --- linux.orig/arch/x86/kernel/apic/x2apic_uv_x.c	2010-07-21 16:53:30.226241589 -0500
> +++ linux/arch/x86/kernel/apic/x2apic_uv_x.c	2010-07-21 16:54:46.358866486 -0500
> @@ -58,6 +58,11 @@ static int uv_is_untracked_pat_range(u64
>  	return is_ISA_range(start, end) || is_GRU_range(start, end);
>  }
>  
> +static int uv_is_wb_acpi_tables(void)
> +{
> +	return 1;
> +}
> +
>  static int early_get_nodeid(void)
>  {
>  	union uvh_node_id_u node_id;
> @@ -81,6 +86,7 @@ static int __init uv_acpi_madt_oem_check
>  		nodeid = early_get_nodeid();
>  		x86_platform.is_untracked_pat_range =  uv_is_untracked_pat_range;
>  		x86_platform.nmi_init = uv_nmi_init;
> +		x86_platform.is_wb_acpi_tables = uv_is_wb_acpi_tables;
>  		if (!strcmp(oem_table_id, "UVL"))
>  			uv_system_type = UV_LEGACY_APIC;
>  		else if (!strcmp(oem_table_id, "UVX"))
> Index: linux/arch/x86/kernel/x86_init.c
> ===================================================================
> --- linux.orig/arch/x86/kernel/x86_init.c	2010-07-21 16:53:30.226241589 -0500
> +++ linux/arch/x86/kernel/x86_init.c	2010-07-21 16:58:17.106240870 -0500
> @@ -71,7 +71,10 @@ struct x86_cpuinit_ops x86_cpuinit __cpu
>  
>  static void default_nmi_init(void) { };
>  
> +static int default_wb_acpi_tables(void) {return 0;}
> +
>  struct x86_platform_ops x86_platform = {
> +	.is_wb_acpi_tables		= default_wb_acpi_tables,
>  	.is_untracked_pat_range		= default_is_untracked_pat_range,
>  	.calibrate_tsc			= native_calibrate_tsc,
>  	.get_wallclock			= mach_get_cmos_time,
> Index: linux/drivers/acpi/osl.c
> ===================================================================
> --- linux.orig/drivers/acpi/osl.c	2010-07-21 16:53:30.226241589 -0500
> +++ linux/drivers/acpi/osl.c	2010-07-21 17:58:20.370414172 -0500
> @@ -293,12 +293,18 @@ acpi_os_map_memory(acpi_physical_address
>  		printk(KERN_ERR PREFIX "Cannot map memory that high\n");
>  		return NULL;
>  	}
> -	if (acpi_gbl_permanent_mmap)
> +	if (acpi_gbl_permanent_mmap) {
>  		/*
>  		* ioremap checks to ensure this is in reserved space
>  		*/
> -		return ioremap((unsigned long)phys, size);
> -	else
> +		if (x86_platform.is_wb_acpi_tables() &&
> +				(e820_all_mapped(phys, phys + size, E820_RAM) ||
> +				e820_all_mapped(phys, phys + size, E820_ACPI) ||
> +				e820_all_mapped(phys, phys + size, E820_NVS)))
> +			return ioremap_cache((unsigned long)phys, size);
> +		else
> +			return ioremap((unsigned long)phys, size);
> +	} else
>  		return __acpi_map_table((unsigned long)phys, size);
>  }
>  EXPORT_SYMBOL_GPL(acpi_os_map_memory);