Hi Shrikanth,
On 17.08.26 09:39, Shrikanth Hegde wrote:
Hi.
In addition to what's currently planned for v11 which was posted here,
https://lore.kernel.org/all/895a058a-475e-42ca-
a7a3-2c854598eea4@linux.ibm.com/
I was going through sashiko's comments at:
https://sashiko.dev/#/patchset/20260812054033.95658-1-
sshegde%40linux.ibm.com
This has revealed some gaps. Thanks to some really nice insights too.
Report quality improving day by day!
Vincent, Dietmar, please check the 32-bit task issue fix on ARM64.
See below.
On 8/12/26 11:10 AM, Shrikanth Hegde wrote:
[...]
Issue1: Possible crash on 32-bit tasks on ARM64.
=======
quoted
quoted
+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;
+
+ return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
+}
Does this intersection check need to account for the architectural CPU
mask?
On asymmetric systems, 32-bit tasks are architecturally restricted by
task_cpu_possible_mask(). If a 32-bit task's mask intersects with
64-bit-only preferred CPUs, this function might return true, causing
is_cpu_allowed() to falsely return false for valid 32-bit non-
preferred CPUs.
Since 64-bit CPUs are rightfully rejected by task_allowed_on_cpu(),
all CPUs
end up rejected. Could this regression cause the select_fallback_rq()
loop
to exhaust all options and hit the BUG() case for 32-bit tasks?
Fix:
====
I wasn;t aware of this case, thanks to sashiko for bring it up.
Yes, it could potentially cause a BUG in select_fallback_rq.
Do a simple check if mask differ from possible mask which indicates we
are on 32-bit task on 64 bit
kernel. Do the below. I think that should solve it.
static inline bool task_can_sched_on_preferred(int cpu, struct
task_struct *p)
{
+ const struct cpumask *valid_mask;
+ int i;
[...]
+ valid_mask = task_cpu_possible_mask(p);
+ if (likely(valid_mask == cpu_possible_mask))
+ return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
+
+ /* 32-bit task */
+ for_each_cpu_and(i, p->cpus_ptr, cpu_preferred_mask) {
+ if (cpumask_test_cpu(i, valid_mask))
+ return true;
+ }
I assume the question is whether task_can_sched_on_preferred() would
have to be changed:
- 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;
so that cpu_preferred_mask can play together nicely with the 'asymmetric
AArch32 EL0 (executing 32-bit Arm userspace under an AArch64 kernel)
support' feature on some mobile Arm64 Socs.
IMHO, this is not necessary since for those tasks p->cpus_ptr is always
a subset of task_cpu_possible_mask(p). 'p->cpus_ptr ∩
cpu_preferred_mask' already cannot contain an architecturally impossible
CPU for those 32-bit Arm userspace tasks.
[...]