Re: [PATCH v11 05/12] sched/core: Try to use a preferred CPU in is_cpu_allowed
From: Shrikanth Hegde
Date: Fri Aug 28 2026 - 07:00:25 EST
Hi Dietmar, Vincent
On 8/28/26 4:08 PM, Vincent Guittot wrote:
On Fri, 28 Aug 2026 at 09:51, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
On 25.08.26 12:38, Shrikanth Hegde wrote:
When possible, try to choose a preferred CPU.
This is essential to maintain user affinities when preferred
CPUs change. A task pinned on a non-preferred CPU should continue
to run there, since this is a non-user triggered event.
If a CPU is non-preferred and the task can run on other CPUs which are
currently preferred, then choose a preferred CPU instead.
This is decided by checking if cpus_ptr and cpu_preferred_mask
intersect or not. If yes, then the task has other preferred CPUs.
The push task mechanism uses a stopper thread which calls
select_fallback_rq() and uses this mechanism to pick a preferred CPU.
This takes care of the wakeup path for FAIR tasks too.
is_cpu_allowed() is called to ensure wakeups happen on preferred CPUs.
With that, additional checks in available_idle_cpu() are not necessary.
Ignore the preferred CPU state if a task's affinity is changing and
its new mask no longer includes the CPU it is currently running on.
This ensures migration_cpu_stop() does not abort, preventing the task
from being stranded outside its allowed affinity.
Account for tasks with architecture-specific CPU masks
(e.g., 32-bit tasks on arm64). For such tasks, explicitly check against
the arch-allowed CPUs to determine if any of the preferred CPUs are
actually valid.
For the majority of cases, this would still keep select_fallback_rq()
as O(N). cpumask_intersects(), which is O(N), is called only if
!cpu_preferred. The task running there is expected to move out.
Subsequently, it should run on a preferred CPU. This becomes O(N**2)
only for tasks pinned solely to non-preferred CPUs. That is a rare case.
Overhead is minimal when the CPU is preferred.
Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
---
kernel/sched/core.c | 41 +++++++++++++++++++++++++++++++++++++++--
1 file changed, 39 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a45f7c308329..f71317fe281d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2494,6 +2494,35 @@ static inline bool rq_has_pinned_tasks(struct rq *rq)
return rq->nr_pinned;
}
+static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p)
+{
+ const struct cpumask *valid_mask;
+ int i;
+
+ if (cpu_preferred(cpu))
+ return false;
+
+ /* Only FAIR tasks honor preferred CPU state */
+ if (unlikely(p->sched_class != &fair_sched_class))
+ return false;
+
+ /* Ignore preferred state if task affinity is changing */
+ if (unlikely(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr)))
+ return false;
+
+ valid_mask = task_cpu_possible_mask(p);
+ if (likely(valid_mask == cpu_possible_mask))
+ return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
+
+ /* Tasks with arch-specific CPU masks. e.g. 32-bit tasks on arm64. */
+ for_each_cpu_and(i, p->cpus_ptr, cpu_preferred_mask) {
+ if (cpumask_test_cpu(i, valid_mask))
+ return true;
+ }
Looking more into this, there might be a window in 64-32-bit execve()
for 32bit EL0 tasks on Arm64 (w/ allow_mismatched_32bit_el0 command line
option).
The time before arch_setup_new_exec() calls
force_compatible_cpus_allowed_ptr() to restrict CPU affinity for those
tasks.
Let me run more test on this ...
Why not simply:
- return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
+ return cpumask_first_and_and(p->cpus_ptr, cpu_preferred_mask,
+ task_cpu_possible_mask(p)) < nr_cpu_ids;
+1
This is the best way to check that there is a valid cpu
Thanks Dietmar, Vincent for checking it further.
It is good to know that there might be a very narrow window.
The three-way cpumask check you suggested will handle that case safely.
IMHO, you want to know whether there is at least one CPU that belongs to
all three CPU masks?
[...]
With that task_can_sched_on_preferred essentially becomes this.
I will put that in v12 and probably send it out after 7.3-rc1 lands.
static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p)
{
if (cpu_preferred(cpu))
return false;
/* Only FAIR tasks honor preferred CPU state */
if (unlikely(p->sched_class != &fair_sched_class))
return false;
/* Ignore preferred state if task affinity is changing */
if (unlikely(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr)))
return false;
return cpumask_first_and_and(p->cpus_ptr, cpu_preferred_mask,
task_cpu_possible_mask(p)) < nr_cpu_ids;
}
Thanks again for checking and for the suggested code!