Re: [PATCH v8 13/13] arm64: topology: divorce MC scheduling domain from core_siblings

[Morten Rasmussen]

On Wed, May 02, 2018 at 05:32:54PM -0500, Jeremy Linton wrote:
> Hi,
> 
> On 05/02/2018 06:49 AM, Morten Rasmussen wrote:
> >On Tue, May 01, 2018 at 03:33:33PM +0100, Sudeep Holla wrote:
> >>
> >>
> >>On 26/04/18 00:31, Jeremy Linton wrote:
> >>>Now that we have an accurate view of the physical topology
> >>>we need to represent it correctly to the scheduler. Generally MC
> >>>should equal the LLC in the system, but there are a number of
> >>>special cases that need to be dealt with.
> >>>
> >>>In the case of NUMA in socket, we need to assure that the sched
> >>>domain we build for the MC layer isn't larger than the DIE above it.
> >>>Similarly for LLC's that might exist in cross socket interconnect or
> >>>directory hardware we need to assure that MC is shrunk to the socket
> >>>or NUMA node.
> >>>
> >>>This patch builds a sibling mask for the LLC, and then picks the
> >>>smallest of LLC, socket siblings, or NUMA node siblings, which
> >>>gives us the behavior described above. This is ever so slightly
> >>>different than the similar alternative where we look for a cache
> >>>layer less than or equal to the socket/NUMA siblings.
> >>>
> >>>The logic to pick the MC layer affects all arm64 machines, but
> >>>only changes the behavior for DT/MPIDR systems if the NUMA domain
> >>>is smaller than the core siblings (generally set to the cluster).
> >>>Potentially this fixes a possible bug in DT systems, but really
> >>>it only affects ACPI systems where the core siblings is correctly
> >>>set to the socket siblings. Thus all currently available ACPI
> >>>systems should have MC equal to LLC, including the NUMA in socket
> >>>machines where the LLC is partitioned between the NUMA nodes.
> >>>
> >>>Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> >>>---
> >>>  arch/arm64/include/asm/topology.h |  2 ++
> >>>  arch/arm64/kernel/topology.c      | 32 +++++++++++++++++++++++++++++++-
> >>>  2 files changed, 33 insertions(+), 1 deletion(-)
> >>>
> >>>diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> >>>index 6b10459e6905..df48212f767b 100644
> >>>--- a/arch/arm64/include/asm/topology.h
> >>>+++ b/arch/arm64/include/asm/topology.h
> >>>@@ -8,8 +8,10 @@ struct cpu_topology {
> >>>  	int thread_id;
> >>>  	int core_id;
> >>>  	int package_id;
> >>>+	int llc_id;
> >>>  	cpumask_t thread_sibling;
> >>>  	cpumask_t core_sibling;
> >>>+	cpumask_t llc_siblings;
> >>>  };
> >>>  extern struct cpu_topology cpu_topology[NR_CPUS];
> >>>diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> >>>index bd1aae438a31..20b4341dc527 100644
> >>>--- a/arch/arm64/kernel/topology.c
> >>>+++ b/arch/arm64/kernel/topology.c
> >>>@@ -13,6 +13,7 @@
> >>>  #include <linux/acpi.h>
> >>>  #include <linux/arch_topology.h>
> >>>+#include <linux/cacheinfo.h>
> >>>  #include <linux/cpu.h>
> >>>  #include <linux/cpumask.h>
> >>>  #include <linux/init.h>
> >>>@@ -214,7 +215,19 @@ EXPORT_SYMBOL_GPL(cpu_topology);
> >>>  const struct cpumask *cpu_coregroup_mask(int cpu)
> >>>  {
> >>>-	return &cpu_topology[cpu].core_sibling;
> >>>+	const cpumask_t *core_mask = cpumask_of_node(cpu_to_node(cpu));
> >>>+
> >>>+	/* Find the smaller of NUMA, core or LLC siblings */
> >>>+	if (cpumask_subset(&cpu_topology[cpu].core_sibling, core_mask)) {
> >>>+		/* not numa in package, lets use the package siblings */
> >>>+		core_mask = &cpu_topology[cpu].core_sibling;
> >>>+	}
> >>>+	if (cpu_topology[cpu].llc_id != -1) {
> >>>+		if (cpumask_subset(&cpu_topology[cpu].llc_siblings, core_mask))
> >>>+			core_mask = &cpu_topology[cpu].llc_siblings;
> >>>+	}
> >>>+
> >>>+	return core_mask;
> >>>  }
> >>>  static void update_siblings_masks(unsigned int cpuid)
> >>>@@ -226,6 +239,9 @@ static void update_siblings_masks(unsigned int cpuid)
> >>>  	for_each_possible_cpu(cpu) {
> >>>  		cpu_topo = &cpu_topology[cpu];
> >>>+		if (cpuid_topo->llc_id == cpu_topo->llc_id)
> >>>+			cpumask_set_cpu(cpu, &cpuid_topo->llc_siblings);
> >>>+
> >>
> >>Would this not result in cpuid_topo->llc_siblings = cpu_possible_mask
> >>on DT systems where llc_id is not set/defaults to -1 and still pass the
> >>condition. Does it make sense to add additional -1 check ?
> >
> >I don't think mask will be used by the current code if llc_id == -1 as
> >the user does the check. Is it better to have the mask empty than
> >default to cpu_possible_mask? If we require all users to implement a
> >check it shouldn't matter.
> >
> 
> Right.
> 
> There is also the other way of thinking about it, which is if you remove the
> if llc_id == -1 check in cpu_coregroup_mask() does it make more sense to
> have llc_siblings default equal all the cores, or just the one being
> requested?

Since we define cpu_coregroup_mask() to be the smallest of LLC, package,
and NUMA node, letting it default to just one cpu would change/break the
topology on non-PPTT systems. Wouldn't it?

If we want to drop the check llc_siblings should be default to either
core_siblings or cpumask_of_node(). But I don't really see the point as
any user of llc_siblings that really care about where the LLC is
would have to check if llc_sibling is just assigned a default value or
it is indeed representing the LLC. I'm fine with just expecting the user
to check llc_id to see if the llc_sibling mask is valid or not.