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FROMLIST: sched: Introduce task_cpu_possible_mask() to limit fallback rq selection

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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>
Bug: 178507149
Link: https://lore.kernel.org/linux-arch/20201208132835.6151-7-will@kernel.org/
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I75985976c196cee7b84043e1a03fcc62f8b6d1c4
This commit is contained in:
Will Deacon
2020-11-12 16:57:39 +00:00
committed by Will Deacon
parent 5ed7a93116
commit 1a96879cb3
2 changed files with 13 additions and 3 deletions
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8
include/linux/mmu_context.h
View File

@@ -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

8
kernel/sched/core.c
View File

@@ -1749,7 +1749,10 @@ 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));
}
/*
@@ -2376,10 +2379,9 @@ 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;