Re: [PATCH v5 8/9] arm64: topology: Enable ACPI/PPTT based CPU topology.

Jeremy Linton
2017-12-15T17:42:09Z

Hi,

On 12/13/2017 12:22 PM, Lorenzo Pieralisi wrote:
> Nit: remove the period in $SUBJECT and capitalize with a coherent
> policy for the patches touching the same code.

Ok, thanks.

>
> On Fri, Dec 01, 2017 at 04:23:29PM -0600, Jeremy Linton wrote:
>> Propagate the topology information from the PPTT tree to the
>> cpu_topology array. We can get the thread id, core_id and
>> cluster_id by assuming certain levels of the PPTT tree correspond
>> to those concepts. The package_id is flagged in the tree and can be
>> found by calling find_acpi_cpu_topology_package() which terminates
>> its search when it finds an ACPI node flagged as the physical
>> package. If the tree doesn't contain enough levels to represent
>> all of the requested levels then the root node will be returned
>> for all subsequent levels.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>> ---
>> arch/arm64/kernel/topology.c | 47 +++++++++++++++++++++++++++++++++++++++++++-
>> include/linux/topology.h | 2 ++
>> 2 files changed, 48 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>> index 74a8a5173a35..198714aca9e8 100644
>> --- a/arch/arm64/kernel/topology.c
>> +++ b/arch/arm64/kernel/topology.c
>> @@ -11,6 +11,7 @@
>> * for more details.
>> */
>>
>> +#include <linux/acpi.h>
>> #include <linux/arch_topology.h>
>> #include <linux/cpu.h>
>> #include <linux/cpumask.h>
>> @@ -22,6 +23,7 @@
>> #include <linux/sched.h>
>> #include <linux/sched/topology.h>
>> #include <linux/slab.h>
>> +#include <linux/smp.h>
>> #include <linux/string.h>
>>
>> #include <asm/cpu.h>
>> @@ -300,6 +302,47 @@ static void __init reset_cpu_topology(void)
>> }
>> }
>>
>> +#ifdef CONFIG_ACPI
>> +/*
>> + * Propagate the topology information of the processor_topology_node tree to the
>> + * cpu_topology array.
>> + */
>> +static int __init parse_acpi_topology(void)
>> +{
>> + u64 is_threaded;
>
> Nit: a bool would be preferable.
>
>> + int cpu;
>> + int topology_id;
>
> int cpu, topology_id;
>
>> + is_threaded = read_cpuid_mpidr() & MPIDR_MT_BITMASK;
>
>> + for_each_possible_cpu(cpu) {
>> + topology_id = find_acpi_cpu_topology(cpu, 0);
>> + if (topology_id < 0)
>> + return topology_id;
>> +
>> + if (is_threaded) {
>> + cpu_topology[cpu].thread_id = topology_id;
>> + topology_id = find_acpi_cpu_topology(cpu, 1);
>> + cpu_topology[cpu].core_id = topology_id;
>> + topology_id = find_acpi_cpu_topology_package(cpu);
>> + cpu_topology[cpu].physical_id = topology_id;
>> + } else {
>> + cpu_topology[cpu].thread_id = -1;
>> + cpu_topology[cpu].core_id = topology_id;
>> + topology_id = find_acpi_cpu_topology_package(cpu);
>> + cpu_topology[cpu].physical_id = topology_id;
>> + }
>> + }
>
> Add a space.
>
> It is probably my fault so apologies if that's the case. The
>
> find_acpi_cpu_topology()
>
> API is a bit strange since it behaves differently according to the
> level passed in.

? In the sense that the id is more directly defined by the firmware for
level0? Not sure this really matters, particularly if at some future
point we come up with a way to standardize an id for the sockets/etc (or
we just renumber the nodes).

AKA, I don't see a problem as we aren't making any guarantees about what
the return id represents other than its unique for siblings at a given
level.

>
> I think it is better to define two calls (it might well have been like
> that in one of the previous series versions):
>
> - find_acpi_cpu_package_level() (returns: package level if success, <0 on
> failure)
> - acpi_cpu_topology_id()

So, knowing the absolute level a given flag is set at allows you to
query a node relative to that level. That is a good idea if you wanted
to identify say a numa in package level (say package-1). But its
complicated by that fact that package-1 may be meaningless in a lot of
cases (maybe package-1 is just a core). The alternative here for finding
a numa proximity domain is to try to find a PPTT node with a subset of
cores which happens to match an proximity domain. But given there is
currently nothing in the specification which requires a 1:1 mapping
between a given set of PPTT nodes and a proximity domain (given the PPTT
is focused on cache nodes) I tend to think we want further flags in the
PPTT and language that makes it clear when this happens. So the
interface should be more generic find_acpi_cpu_flag_level(cpu, flag).
That way we avoid the complexity of trying to guess from a relative
level if a particular topological feature is appropriate.

>
> It would even be better to lump the two calls together but you do not
> know how many topology levels are there so it becomes a bit complicated
> to handle.

Right, which is basically what the underlying
find_acpi_cpu_topology_tag() is doing at the moment. My tendency here is
just to export that call and let the PPTT parsing code wrap the decision
about what to do if the flag can't be found. Which means the whole
discussion above about letting the higher level code try to infer
relative levels is a last resort. I'm more for creating a couple
additional flags (FLAG_GIVE_ME_LEVEL_WHERE_NUMA_STARTS) and feeding it
to acpi_cpu_topology_tag() with some additional logic which says
something to the affect, return the NUMA level in the PPTT described by
some future standardized flag, otherwise return the socket level, and if
that doesn't exist return the root node level (or maybe if we get
desperate a node which seems to match a SRAT proximity domain). That
keeps the PPTT interface fairly simple, and keeps the level selection
out of the common topology code.

There are also some other possible future directions which don't fit any
of these models. So in that regard I think we want to keep the inteface
as simple as possible, and transform it in the future when we have a
direct need for it.

Thanks,