Re: [PATCH 2/2] x86, acpi: Handle apic/x2apic entries in MADT in correct order

[Lorenzo Pieralisi]

On Wed, Sep 09, 2015 at 10:30:18AM +0100, Lukasz Anaczkowski wrote:
> ACPI specifies the following rules when listing APIC IDs:
> (1) Boot processor is listed first
> (2) For multi-threaded processors, BIOS should list the first logical
>     processor of each of the individual multi-threaded processors in MADT
>     before listing any of the second logical processors.
> (3) APIC IDs < 0xFF should be listed in APIC subtable, APIC IDs >= 0xFF
>     should be listed in X2APIC subtable
> 
> Because of above, when there's more than 0xFF logical CPUs, BIOS
> interleaves APIC/X2APIC subtables.
> 
> Assuming, there's 72 cores, 72 hyper-threads each, 288 CPUs total,
> listing is like this:
> 
> APIC (0,4,8, .., 252)
> X2APIC (258,260,264, .. 284)
> APIC (1,5,9,...,253)
> X2APIC (259,261,265,...,285)
> APIC (2,6,10,...,254)
> X2APIC (260,262,266,..,286)
> APIC (3,7,11,...,251)
> X2APIC (255,261,262,266,..,287)
> 
> Now, before this patch, due to how ACPI MADT subtables were parsed (BSP
> then X2APIC then APIC), kernel enumerated CPUs in reverted order (i.e.
> high APIC IDs were getting low logical IDs, and low APIC IDs were
> getting high logical IDs).
> This is wrong for the following reasons:
> () it's hard to predict how cores and threads are enumerated

So ? Why would the logical cpus order matters at all ? I guessed
there are probeable properties that allows the kernel to build
the affinity (ie topology list, shared caches, smt siblings, etc).
Please explain, since I am confused, to me all you need is a list
of existing physical ids, in whatever order they come, that's at least
what we need on ARM.

> () when it's hard to predict, s/w threads cannot be properly affinitized
>    causing significant performance impact due to e.g. inproper cache
>    sharing

See above. Why cpus logical ordering would matter here ?

> () enumeration is inconsistent with how threads are enumerated on
>    other Intel Xeon processors

And why does that matter ? Is it because userspace is making assumptions
on the logical cpu enumeration scheme ? I am just asking, I would
like to understand.

> So, order in which MADT APIC/X2APIC handlers are passed is
> reverse and both handlers are passed to be called during same MADT
> table to walk to achieve correct CPU enumeration.

Define "correct" please, you define the logical ordering you
want to achieve, you do not define why that's more "correct"
than the current implementation.

Thank you,
Lorenzo

> In scenario when someone boots kernel with options 'maxcpus=72 nox2apic',
> in result less cores may be booted, since some of the CPUs the kernel
> will try to use will have APIC ID >= 0xFF. In such case, one
> should not pass 'nox2apic'.
> 
> Disclimer: code parsing MADT APIC/X2APIC has not been touched since 2009,
> when X2APIC support was initially added. I do not know why MADT parsing
> code was added in the reversed order in the first place.
> I guess it didn't matter at that time since nobody cared about cores
> with APIC IDs >= 0xFF, right?
> 
> This patch is based on work of "Yinghai Lu <yinghai@xxxxxxxxxx>"
> previously published at https://lkml.org/lkml/2013/1/21/563
> 
> Here's the explanation why parsing interface needs to be changed
> and why simpler approach will not work https://lkml.org/lkml/2015/9/7/285
> 
> Signed-off-by: Lukasz Anaczkowski <lukasz.anaczkowski@xxxxxxxxx>
> Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> (commit message)
> ---
>  arch/x86/kernel/acpi/boot.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index e49ee24..116e911 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -981,6 +981,8 @@ static int __init acpi_parse_madt_lapic_entries(void)
>  {
>  	int count;
>  	int x2count = 0;
> +	int ret;
> +	struct acpi_subtable_proc madt_proc[2];
>  
>  	if (!cpu_has_apic)
>  		return -ENODEV;
> @@ -1004,10 +1006,22 @@ static int __init acpi_parse_madt_lapic_entries(void)
>  				      acpi_parse_sapic, MAX_LOCAL_APIC);
>  
>  	if (!count) {
> -		x2count = acpi_table_parse_madt(ACPI_MADT_TYPE_LOCAL_X2APIC,
> -					acpi_parse_x2apic, MAX_LOCAL_APIC);
> -		count = acpi_table_parse_madt(ACPI_MADT_TYPE_LOCAL_APIC,
> -					acpi_parse_lapic, MAX_LOCAL_APIC);
> +		memset(madt_proc, 0, sizeof(madt_proc));
> +		madt_proc[0].id = ACPI_MADT_TYPE_LOCAL_APIC;
> +		madt_proc[0].handler = acpi_parse_lapic;
> +		madt_proc[1].id = ACPI_MADT_TYPE_LOCAL_X2APIC;
> +		madt_proc[1].handler = acpi_parse_x2apic;
> +		ret = acpi_table_parse_entries_array(ACPI_SIG_MADT,
> +				sizeof(struct acpi_table_madt),
> +				madt_proc, ARRAY_SIZE(madt_proc), MAX_LOCAL_APIC);
> +		if (ret < 0) {
> +			printk(KERN_ERR PREFIX
> +					"Error parsing LAPIC/X2APIC entries\n");
> +			return ret;
> +		}
> +
> +		x2count = madt_proc[0].count;
> +		count = madt_proc[1].count;
>  	}
>  	if (!count && !x2count) {
>  		printk(KERN_ERR PREFIX "No LAPIC entries present\n");
> -- 
> 1.8.3.1
> 
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