Re: [PATCH part1 0/5] acpica: Split acpi_gbl_root_table_list initialization into two parts.

[Rafael J. Wysocki]

On Thursday, August 08, 2013 11:39:31 AM Tang Chen wrote:
> [Problem]
> 
> The current Linux cannot migrate pages used by the kerenl because
> of the kernel direct mapping. In Linux kernel space, va = pa + PAGE_OFFSET.
> When the pa is changed, we cannot simply update the pagetable and
> keep the va unmodified. So the kernel pages are not migratable.
> 
> There are also some other issues will cause the kernel pages not migratable.
> For example, the physical address may be cached somewhere and will be used.
> It is not to update all the caches.
> 
> When doing memory hotplug in Linux, we first migrate all the pages in one
> memory device somewhere else, and then remove the device. But if pages are
> used by the kernel, they are not migratable. As a result, memory used by
> the kernel cannot be hot-removed.
> 
> Modifying the kernel direct mapping mechanism is too difficult to do. And
> it may cause the kernel performance down and unstable. So we use the following
> way to do memory hotplug.
> 
> 
> [What we are doing]
> 
> In Linux, memory in one numa node is divided into several zones. One of the
> zones is ZONE_MOVABLE, which the kernel won't use.
> 
> In order to implement memory hotplug in Linux, we are going to arrange all
> hotpluggable memory in ZONE_MOVABLE so that the kernel won't use these memory.
> 
> To do this, we need ACPI's help.
> 
> 
> [How we do this]
> 
> In ACPI, SRAT(System Resource Affinity Table) contains NUMA info. The memory
> affinities in SRAT record every memory range in the system, and also, flags
> specifying if the memory range is hotpluggable.
> (Please refer to ACPI spec 5.0 5.2.16)
> 
> With the help of SRAT, we have to do the following two things to achieve our
> goal:
> 
> 1. When doing memory hot-add, allow the users arranging hotpluggable as
>    ZONE_MOVABLE.
>    (This has been done by the MOVABLE_NODE functionality in Linux.)
> 
> 2. when the system is booting, prevent bootmem allocator from allocating
>    hotpluggable memory for the kernel before the memory initialization
>    finishes.
>    (This is what we are going to do. And we need to do some modification in
>     ACPICA. See below.)
> 
> 
> [About this patch-set]
> 
> There is a bootmem allocator named memblock in Linux. memblock starts to work
> at very early time, and SRAT has not been parsed. So we don't know which memory
> is hotpluggable. In order to prevent memblock from allocating hotpluggable
> memory for the kernel, we need to obtain SRAT memory affinity info earlier.
> 
> In the current Linux kernel, the acpica code iterates acpi_gbl_root_table_list,
> and install all the acpi tables into it at boot time. Then, it tries to find
> if there is any override table in global array acpi_tables_addr. If any, reinstall
> the override table into acpi_gbl_root_table_list.
> 
> In Linux, global array acpi_tables_addr can be fulfilled by ACPI_INITRD_TABLE_OVERRIDE
> mechanism, which allows users to specify their own ACPI tables in initrd file, and
> override the ones from firmware.
> 
> The whole procedure looks like the following:
> 
> setup_arch()
>  |->   ......                                     /* Setup direct mapping pagetables */
>  |->acpi_initrd_override()                        /* Store all override tables in acpi_tables_addr. */
>  |...
>  |->acpi_boot_table_init()
>     |->acpi_table_init()
>        |                                                                                  (Linux code)
> ......................................................................................................
>        |                                                                                 (ACPICA code)
>        |->acpi_initialize_tables()
>           |->acpi_tb_parse_root_table()           /* Parse RSDT or XSDT, find all tables in firmware */
>              |->for (each item in acpi_gbl_root_table_list)
>                 |->acpi_tb_install_table()
>                    |->   ......                   /* Install one single table */
>                    |->acpi_tb_table_override()    /* Override one single table */
> 
> It does the table installation and overriding one by one.
> 
> In order to find SRAT at earlier time, we want to initialize acpi_gbl_root_table_list
> earlier. But at the same time, keep ACPI_INITRD_TABLE_OVERRIDE procedure works as well.
> 
> The basic idea is, split the acpi_gbl_root_table_list initialization procedure into
> two steps:
> 1. Install all tables from firmware, not one by one.
> 2. Override any table if necessary, not one by one.
> 
> After this patch-set, it will work like this:
> 
> setup_arch()
>  |->     ......                                   /* Install all tables from firmware (Step 1) */
>  |->     ......                                   /* Try to find if any override SRAT in initrd file, if yes, use it */
>  |->     ......                                   /* Use the SRAT from firmware */
>  |->     ......                                   /* memblock starts to work */
>  |->     ......
>  |->acpi_initrd_override()                        /* Initialize acpi_tables_addr with all override table. */
>  |...
>  |->     ......                                   /* Do the table override work for all tables (Step 2) */
> 
> 
> In order to achieve this goal, we have to split all the following functions:
> 
> ACPICA:
>     acpi_tb_install_table()
>     acpi_tb_parse_root_table()
>     acpi_initialize_tables()
> 
> Linux acpi:
>     acpi_table_init()
>     acpi_boot_table_init()
> 
> Since ACPICA code is not just used by the Linux, so we should keep the ACPICA
> side interfaces unmodified, and introduce new functions used in Linux.
> 
> 
> Tang Chen (5):
>   acpi, acpica: Split acpi_tb_install_table() into two parts.
>   acpi, acpica: Call two new functions instead of
>     acpi_tb_install_table() in acpi_tb_parse_root_table().
>   acpi, acpica: Split acpi_tb_parse_root_table() into two parts.
>   acpi, acpica: Call two new functions instead of
>     acpi_tb_parse_root_table() in acpi_initialize_tables().
>   acpi, acpica: Split acpi_initialize_tables() into two parts.
> 
>  drivers/acpi/acpica/actables.h |    2 +
>  drivers/acpi/acpica/tbutils.c  |  184 +++++++++++++++++++++++++++++++++++-----
>  drivers/acpi/acpica/tbxface.c  |   69 ++++++++++++++--
>  3 files changed, 228 insertions(+), 27 deletions(-)

OK, thanks for re-organizing the patch series!

Bob, Lv, can you please review this part and let us know what you think?

Rafael

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