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Subject[tip: sched/urgent] sched,x86: Don't use cluster topology for x86 hybrid CPUs
The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: cabdc3a8475b918e55744f43719b26a82dc8fa6b
Gitweb: https://git.kernel.org/tip/cabdc3a8475b918e55744f43719b26a82dc8fa6b
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Sat, 04 Dec 2021 10:14:02 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 08 Dec 2021 22:15:37 +01:00

sched,x86: Don't use cluster topology for x86 hybrid CPUs

For x86 hybrid CPUs like Alder Lake, the order of CPU selection should
be based strictly on CPU priority. Don't include cluster topology for
hybrid CPUs to avoid interference with such CPU selection order.

On Alder Lake, the Atom CPU cluster has more capacity (4 Atom CPUs) vs
Big core cluster (2 hyperthread CPUs). This could potentially bias CPU
selection towards Atom over Big Core, when Big core CPU has higher
priority.

Fixes: 66558b730f25 ("sched: Add cluster scheduler level for x86")
Suggested-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Link: https://lkml.kernel.org/r/20211204091402.GM16608@worktop.programming.kicks-ass.net
---
arch/x86/kernel/smpboot.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index ac2909f..617012f 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -579,6 +579,17 @@ static struct sched_domain_topology_level x86_numa_in_package_topology[] = {
{ NULL, },
};

+static struct sched_domain_topology_level x86_hybrid_topology[] = {
+#ifdef CONFIG_SCHED_SMT
+ { cpu_smt_mask, x86_smt_flags, SD_INIT_NAME(SMT) },
+#endif
+#ifdef CONFIG_SCHED_MC
+ { cpu_coregroup_mask, x86_core_flags, SD_INIT_NAME(MC) },
+#endif
+ { cpu_cpu_mask, SD_INIT_NAME(DIE) },
+ { NULL, },
+};
+
static struct sched_domain_topology_level x86_topology[] = {
#ifdef CONFIG_SCHED_SMT
{ cpu_smt_mask, x86_smt_flags, SD_INIT_NAME(SMT) },
@@ -1469,8 +1480,11 @@ void __init native_smp_cpus_done(unsigned int max_cpus)

calculate_max_logical_packages();

+ /* XXX for now assume numa-in-package and hybrid don't overlap */
if (x86_has_numa_in_package)
set_sched_topology(x86_numa_in_package_topology);
+ if (cpu_feature_enabled(X86_FEATURE_HYBRID_CPU))
+ set_sched_topology(x86_hybrid_topology);

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