Re: [PATCH v3] topology: make core_mask include at least cluster_siblings

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 09/03/2022 19:26, Darren Hart wrote:
> On Wed, Mar 09, 2022 at 01:50:07PM +0100, Dietmar Eggemann wrote:
>> On 08/03/2022 18:49, Darren Hart wrote:
>>> On Tue, Mar 08, 2022 at 05:03:07PM +0100, Dietmar Eggemann wrote:
>>>> On 08/03/2022 12:04, Vincent Guittot wrote:
>>>>> On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@kernel.org> wrote:

[...]

>>>> I do not have any better idea than this tweak here either in case the
>>>> platform can't provide a cleaner setup.
>>>
>>> I'd argue The platform is describing itself accurately in ACPI PPTT
>>> terms. The topology doesn't fit nicely within the kernel abstractions
>>> today. This is an area where I hope to continue to improve things going
>>> forward.
>>
>> I see. And I assume lying about SCU/LLC boundaries in ACPI is not an
>> option since it messes up /sys/devices/system/cpu/cpu0/cache/index*/.
>>
>> [...]
> 
> I'm not aware of a way to accurately describe the SCU topology in the PPTT, and
> the risk we run with lying about LLC topology is that lie has to be comprehended
> by all OSes and not conflict with other lies people may ask for. In general, I
> think it is preferable and more maintainable to describe the topology as
> accurately and honestly as we can within the existing platform mechanisms (PPTT,
> HMAT, etc) and work on the higher level abstractions to accommodate a broader
> set of topologies as they emerge (as well as working to more fully describe the
> topology with new platform level mechanisms as needed).
> 
> As I mentioned, I intend to continue looking in to how to improve the current
> abstractions. For now, it sounds like we have agreement that this patch can be
> merged to address the BUG?

What about swapping the CLS and MC cpumasks for such a machine? This
would avoid that the task scheduler has to deal with a system which has
CLS but no MC. We essentially promote the CLS cpumask up to MC in this
case.

cat /sys/kernel/debug/sched/domains/cpu0/domain*/name
MC
^^
DIE
NUMA

cat /sys/kernel/debug/sched/domains/cpu0# cat domain*/flags
SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
                                                                  ^^^^^^^^^^^^^^^^^^^^^^ 
SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_PREFER_SIBLING 
SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA

Only very lightly tested on Altra and Juno-r0 (DT).

--->8---

From 54bef59e7f50fa41b7ae39190fd71af57209c27d Mon Sep 17 00:00:00 2001
From: Dietmar Eggemann <dietmar.eggemann@arm.com>
Date: Mon, 14 Mar 2022 15:08:23 +0000
Subject: [PATCH] arch_topology: Swap MC & CLS SD mask if MC weight==1 &
 subset(MC,CLS)

This avoids the issue of having a system with a CLS SD but no MC SD.
CLS should be sub-SD of MC.

The cpumask under /sys/devices/system/cpu/cpu*/cache/index* and
/sys/devices/system/cpu/cpu*/topology are not changed by this.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 drivers/base/arch_topology.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 976154140f0b..9af90a5625c7 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -614,7 +614,7 @@ static int __init parse_dt_topology(void)
 struct cpu_topology cpu_topology[NR_CPUS];
 EXPORT_SYMBOL_GPL(cpu_topology);
 
-const struct cpumask *cpu_coregroup_mask(int cpu)
+const struct cpumask *_cpu_coregroup_mask(int cpu)
 {
 	const cpumask_t *core_mask = cpumask_of_node(cpu_to_node(cpu));
 
@@ -631,11 +631,37 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
 	return core_mask;
 }
 
-const struct cpumask *cpu_clustergroup_mask(int cpu)
+const struct cpumask *_cpu_clustergroup_mask(int cpu)
 {
 	return &cpu_topology[cpu].cluster_sibling;
 }
 
+static int
+swap_masks(const cpumask_t *core_mask, const cpumask_t *cluster_mask)
+{
+	if (cpumask_weight(core_mask) == 1 &&
+	    cpumask_subset(core_mask, cluster_mask))
+		return 1;
+
+	return 0;
+}	
+
+const struct cpumask *cpu_coregroup_mask(int cpu)
+{
+	const cpumask_t *cluster_mask = _cpu_clustergroup_mask(cpu);
+	const cpumask_t *core_mask = _cpu_coregroup_mask(cpu);
+	
+	return swap_masks(core_mask, cluster_mask) ? cluster_mask : core_mask;
+}
+
+const struct cpumask *cpu_clustergroup_mask(int cpu)
+{
+	const cpumask_t *cluster_mask = _cpu_clustergroup_mask(cpu);
+	const cpumask_t *core_mask = _cpu_coregroup_mask(cpu);
+
+	return swap_masks(core_mask, cluster_mask) ? core_mask : cluster_mask;
+}
+
 void update_siblings_masks(unsigned int cpuid)
 {
 	struct cpu_topology *cpu_topo, *cpuid_topo = &cpu_topology[cpuid];
-- 
2.25.1

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