[PATCH 08/10] arm64: make cpu number a percpu variable
Mark Rutland
mark.rutland at arm.com
Wed Oct 19 12:10:32 PDT 2016
In the absence of CONFIG_THREAD_INFO_IN_TASK, core code maintains
thread_info::cpu, and low-level architecture code can access this to
build raw_smp_processor_id(). With CONFIG_THREAD_INFO_IN_TASK, core code
maintains task_struct::cpu, which for reasons of hte header soup is not
accessible to low-level arch code.
Instead, we can maintain a percpu variable containing the cpu number.
Non-preemptible code will receive the current CPU's number, while
preemptible code may race (as was previosuly the case), and see a stale
value. Thus, there shouldn't be a change in observable behaviour.
For both the old and new implementation of raw_smp_processor_id(), we
read a syreg into a GPR, add an offset, and load the result. As the
offset is now large, it probably won't be folded into the load, but
otherwise the assembly shouldn't change much.
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: James Morse <james.morse at arm.com>
Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
---
arch/arm64/include/asm/smp.h | 6 +++++-
arch/arm64/kernel/smp.c | 5 +++++
2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index 0226447..f77ac0d 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -29,11 +29,15 @@
#ifndef __ASSEMBLY__
+#include <asm/percpu.h>
+
#include <linux/threads.h>
#include <linux/cpumask.h>
#include <linux/thread_info.h>
-#define raw_smp_processor_id() (current_thread_info()->cpu)
+DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number);
+
+#define raw_smp_processor_id() (*this_cpu_ptr(&cpu_number))
struct seq_file;
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 7af46bf..2679722 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -58,6 +58,9 @@
#define CREATE_TRACE_POINTS
#include <trace/events/ipi.h>
+DEFINE_PER_CPU_READ_MOSTLY(int, cpu_number);
+EXPORT_PER_CPU_SYMBOL(cpu_number);
+
/*
* as from 2.5, kernels no longer have an init_tasks structure
* so we need some other way of telling a new secondary core
@@ -718,6 +721,8 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
*/
for_each_possible_cpu(cpu) {
+ per_cpu(cpu_number, cpu) = cpu;
+
if (cpu == smp_processor_id())
continue;
--
1.9.1
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