lkml.org 
[lkml]   [2020]   [Dec]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v5 06/15] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection
Date
Asymmetric systems may not offer the same level of userspace ISA support
across all CPUs, meaning that some applications cannot be executed by
some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
not feature support for 32-bit applications on both clusters.

On such a system, we must take care not to migrate a task to an
unsupported CPU when forcefully moving tasks in select_fallback_rq()
in response to a CPU hot-unplug operation.

Introduce a task_cpu_possible_mask() hook which, given a task argument,
allows an architecture to return a cpumask of CPUs that are capable of
executing that task. The default implementation returns the
cpu_possible_mask, since sane machines do not suffer from per-cpu ISA
limitations that affect scheduling. The new mask is used when selecting
the fallback runqueue as a last resort before forcing a migration to the
first active CPU.

Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
include/linux/mmu_context.h | 8 ++++++++
kernel/sched/core.c | 8 +++++---
2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
index 03dee12d2b61..bc4ac3c525e6 100644
--- a/include/linux/mmu_context.h
+++ b/include/linux/mmu_context.h
@@ -14,4 +14,12 @@
static inline void leave_mm(int cpu) { }
#endif

+/*
+ * CPUs that are capable of running task @p. By default, we assume a sane,
+ * homogeneous system. Must contain at least one active CPU.
+ */
+#ifndef task_cpu_possible_mask
+# define task_cpu_possible_mask(p) cpu_possible_mask
+#endif
+
#endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e7e453492cff..58474569a2ea 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1708,7 +1708,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
if (is_per_cpu_kthread(p))
return cpu_online(cpu);

- return cpu_active(cpu);
+ if (!cpu_active(cpu))
+ return false;
+
+ return cpumask_test_cpu(cpu, task_cpu_possible_mask(p));
}

/*
@@ -2318,10 +2321,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
}
fallthrough;
case possible:
- do_set_cpus_allowed(p, cpu_possible_mask);
+ do_set_cpus_allowed(p, task_cpu_possible_mask(p));
state = fail;
break;
-
case fail:
BUG();
break;
--
2.29.2.576.ga3fc446d84-goog
\
 
 \ /
  Last update: 2020-12-08 14:31    [W:0.222 / U:0.208 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site