[PATCH] kthread: fall back when no preferred housekeeping CPU is online

[PATCH] kthread: fall back when no preferred housekeeping CPU is online

[Avi Weiss]

kthread_fetch_affinity() falls back to the housekeeping mask
only when the intersection with the preferred mask is empty.

If the intersection contains only offline CPUs,
set_cpus_allowed_ptr() returns -EINVAL and the kthread keeps
its previous affinity, which may still include an isolated CPU.

Check whether the intersection contains an online CPU instead.
Fall back to the full housekeeping mask when none are online.

Fixes: e894f6339808 ("kthread: Honour kthreads preferred affinity after cpuset changes")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
---

Tested on x86_64 Linux 7.3.0 with vng/qemu and a kthread with 
preferred CPUs 1-2.

With CPU 2 offline and CPU 1 isolated, the unpatched kernel kept
affinity 1-2 and set_cpus_allowed_ptr() returned -EINVAL. With the
fix, affinity changed to 0,2. Bringing CPU 2 online restored 
affinity to 2, and removing isolation restored 1-2.

 kernel/kthread.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index cc8cb5d3eab7..fdb8fb07f40b 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -343,7 +343,7 @@ static void kthread_fetch_affinity(struct kthread *kthread, struct cpumask *cpum
 	}
 
 	cpumask_and(cpumask, pref, housekeeping_cpumask(HK_TYPE_DOMAIN));
-	if (cpumask_empty(cpumask))
+	if (!cpumask_intersects(cpumask, cpu_online_mask))
 		cpumask_copy(cpumask, housekeeping_cpumask(HK_TYPE_DOMAIN));
 }
 

base-commit: f47d223627f80cb7a1eb14c585fe6cb35de5e984
-- 
2.43.0

Re: [PATCH] kthread: fall back when no preferred housekeeping CPU is online

[Bradley Morgan]

On 20 October 2026 13:55:18 BST, Avi Weiss <thnkslprpt@gmail.com> wrote:
>kthread_fetch_affinity() falls back to the housekeeping mask
>only when the intersection with the preferred mask is empty.
>
>If the intersection contains only offline CPUs,
>set_cpus_allowed_ptr() returns -EINVAL and the kthread keeps
>its previous affinity, which may still include an isolated CPU.
>
>Check whether the intersection contains an online CPU instead.
>Fall back to the full housekeeping mask when none are online.
>
>Fixes: e894f6339808 ("kthread: Honour kthreads preferred affinity after cpuset changes")
>Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
>---
>
>Tested on x86_64 Linux 7.3.0 with vng/qemu and a kthread with 
>preferred CPUs 1-2.
>
>With CPU 2 offline and CPU 1 isolated, the unpatched kernel kept
>affinity 1-2 and set_cpus_allowed_ptr() returned -EINVAL. With the
>fix, affinity changed to 0,2. Bringing CPU 2 online restored 
>affinity to 2, and removing isolation restored 1-2.
>
> kernel/kthread.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/kernel/kthread.c b/kernel/kthread.c
>index cc8cb5d3eab7..fdb8fb07f40b 100644
>--- a/kernel/kthread.c
>+++ b/kernel/kthread.c
>@@ -343,7 +343,7 @@ static void kthread_fetch_affinity(struct kthread *kthread, struct cpumask *cpum
> 	}
> 
> 	cpumask_and(cpumask, pref, housekeeping_cpumask(HK_TYPE_DOMAIN));
>-	if (cpumask_empty(cpumask))
>+	if (!cpumask_intersects(cpumask, cpu_online_mask))

Nice catch! 

Reviewed-by: Bradley Morgan <brads@mainlining.org>


> 		cpumask_copy(cpumask, housekeeping_cpumask(HK_TYPE_DOMAIN));
> }
> 
>
>base-commit: f47d223627f80cb7a1eb14c585fe6cb35de5e984
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds