* [PATCH 2/2] fs/proc: Show the content of task->user_cpus_ptr in /proc/<pid>/status
2025-09-23 17:54 [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity() Waiman Long
@ 2025-09-23 17:54 ` Waiman Long
2025-10-20 20:06 ` [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity() Waiman Long
2025-10-20 20:13 ` David Hildenbrand
2 siblings, 0 replies; 5+ messages in thread
From: Waiman Long @ 2025-09-23 17:54 UTC (permalink / raw)
To: Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot,
Dietmar Eggemann, Steven Rostedt, Ben Segall, Mel Gorman,
Valentin Schneider, Jonathan Corbet
Cc: linux-kernel, linux-fsdevel, linux-doc, Andrew Morton,
David Hildenbrand, Catalin Marinas, Nico Pache, Phil Auld,
John Coleman, Waiman Long
The task->user_cpus_ptr was introduced by commit b90ca8badbd1 ("sched:
Introduce task_struct::user_cpus_ptr to track requested affinity") to
keep track of user-requested CPU affinity. With commit da019032819a
("sched: Enforce user requested affinity"), user_cpus_ptr will
persistently affect how cpus_allowed will be set. So it makes sense to
enable users to see the presence of a previously set user_cpus_ptr so
they can do something about it without getting a surprise.
Add new "Cpus_user" and "Cpus_user_list" fields to /proc/<pid>/status
output via task_cpus_allowed() as the presence of user_cpus_ptr will
affect the cpus_allowed cpumask.
Signed-off-by: Waiman Long <longman@redhat.com>
---
Documentation/filesystems/proc.rst | 2 ++
fs/proc/array.c | 9 +++++++++
2 files changed, 11 insertions(+)
diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 2971551b7235..fb9e7753010c 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -311,6 +311,8 @@ It's slow but very precise.
SpeculationIndirectBranch indirect branch speculation mode
Cpus_allowed mask of CPUs on which this process may run
Cpus_allowed_list Same as previous, but in "list format"
+ Cpus_user mask of user requested CPUs from sched_setaffinity(2)
+ Cpus_user_list Same as previous, but in "list format"
Mems_allowed mask of memory nodes allowed to this process
Mems_allowed_list Same as previous, but in "list format"
voluntary_ctxt_switches number of voluntary context switches
diff --git a/fs/proc/array.c b/fs/proc/array.c
index d6a0369caa93..30ceab935e13 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -405,10 +405,19 @@ static inline void task_context_switch_counts(struct seq_file *m,
static void task_cpus_allowed(struct seq_file *m, struct task_struct *task)
{
+ cpumask_t *user_cpus = task->user_cpus_ptr;
+
seq_printf(m, "Cpus_allowed:\t%*pb\n",
cpumask_pr_args(&task->cpus_mask));
seq_printf(m, "Cpus_allowed_list:\t%*pbl\n",
cpumask_pr_args(&task->cpus_mask));
+
+ if (user_cpus) {
+ seq_printf(m, "Cpus_user:\t%*pb\n", cpumask_pr_args(user_cpus));
+ seq_printf(m, "Cpus_user_list:\t%*pbl\n", cpumask_pr_args(user_cpus));
+ } else {
+ seq_puts(m, "Cpus_user:\nCpus_user_list:\n");
+ }
}
static inline void task_core_dumping(struct seq_file *m, struct task_struct *task)
--
2.51.0
^ permalink raw reply related [flat|nested] 5+ messages in thread* Re: [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity()
2025-09-23 17:54 [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity() Waiman Long
2025-09-23 17:54 ` [PATCH 2/2] fs/proc: Show the content of task->user_cpus_ptr in /proc/<pid>/status Waiman Long
@ 2025-10-20 20:06 ` Waiman Long
2025-10-20 20:13 ` David Hildenbrand
2 siblings, 0 replies; 5+ messages in thread
From: Waiman Long @ 2025-10-20 20:06 UTC (permalink / raw)
To: Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot,
Dietmar Eggemann, Steven Rostedt, Ben Segall, Mel Gorman,
Valentin Schneider, Jonathan Corbet
Cc: linux-kernel, linux-fsdevel, linux-doc, Andrew Morton,
David Hildenbrand, Catalin Marinas, Nico Pache, Phil Auld,
John Coleman
On 9/23/25 1:54 PM, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> perserved even if the task is being moved to a different cpuset.
> However, that affinity is also being inherited by any subsequently
> created child processes which may not want or be aware of that affinity.
>
> One way to solve this problem is to provide a way to back off from
> that user provided CPU affinity. This patch implements such a scheme
> by using a full cpumask (a cpumask with all bits set) to signal the
> clearing of the user cpumask to follow the default as allowed by
> the current cpuset. In fact, with a full cpumask in user_cpus_ptr,
> the task behavior should be the same as with a NULL user_cpus_ptr.
> This patch just formalizes it without causing any incompatibility and
> discard an otherwise useless cpumask.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> kernel/sched/syscalls.c | 20 ++++++++++++++------
> 1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 77ae87f36e84..d68c7a4ee525 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1229,14 +1229,22 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
> return retval;
>
> /*
> - * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> - * alloc_user_cpus_ptr() returns NULL.
> + * If a full cpumask is passed in, clear user_cpus_ptr and reset the
> + * current cpu affinity to the default for the current cpuset.
> */
> - user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> - if (user_mask) {
> - cpumask_copy(user_mask, in_mask);
> + if (cpumask_full(in_mask)) {
> + user_mask = NULL;
> } else {
> - return -ENOMEM;
> + /*
> + * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> + * alloc_user_cpus_ptr() returns NULL.
> + */
> + user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> + if (user_mask) {
> + cpumask_copy(user_mask, in_mask);
> + } else {
> + return -ENOMEM;
> + }
> }
>
> ac = (struct affinity_context){
Any comment or suggested improvement on this patch and the following one?
Thanks,
Longman
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity()
2025-09-23 17:54 [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity() Waiman Long
2025-09-23 17:54 ` [PATCH 2/2] fs/proc: Show the content of task->user_cpus_ptr in /proc/<pid>/status Waiman Long
2025-10-20 20:06 ` [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity() Waiman Long
@ 2025-10-20 20:13 ` David Hildenbrand
2025-10-20 20:21 ` Waiman Long
2 siblings, 1 reply; 5+ messages in thread
From: David Hildenbrand @ 2025-10-20 20:13 UTC (permalink / raw)
To: Waiman Long, Ingo Molnar, Peter Zijlstra, Juri Lelli,
Vincent Guittot, Dietmar Eggemann, Steven Rostedt, Ben Segall,
Mel Gorman, Valentin Schneider, Jonathan Corbet
Cc: linux-kernel, linux-fsdevel, linux-doc, Andrew Morton,
Catalin Marinas, Nico Pache, Phil Auld, John Coleman
On 23.09.25 19:54, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> perserved even if the task is being moved to a different cpuset.
> However, that affinity is also being inherited by any subsequently
> created child processes which may not want or be aware of that affinity.
So I assume setting the affinity to the full bitmap would then allow any
child to essentially reset to the default, correct?
>
> One way to solve this problem is to provide a way to back off from
> that user provided CPU affinity. This patch implements such a scheme
> by using a full cpumask (a cpumask with all bits set) to signal the
> clearing of the user cpumask to follow the default as allowed by
> the current cpuset. In fact, with a full cpumask in user_cpus_ptr,
> the task behavior should be the same as with a NULL user_cpus_ptr.
> This patch just formalizes it without causing any incompatibility and
> discard an otherwise useless cpumask.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> kernel/sched/syscalls.c | 20 ++++++++++++++------
> 1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 77ae87f36e84..d68c7a4ee525 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1229,14 +1229,22 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
> return retval;
>
> /*
> - * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> - * alloc_user_cpus_ptr() returns NULL.
> + * If a full cpumask is passed in, clear user_cpus_ptr and reset the
> + * current cpu affinity to the default for the current cpuset.
> */
> - user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> - if (user_mask) {
> - cpumask_copy(user_mask, in_mask);
> + if (cpumask_full(in_mask)) {
> + user_mask = NULL;
> } else {
> - return -ENOMEM;
> + /*
> + * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> + * alloc_user_cpus_ptr() returns NULL.
> + */
> + user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> + if (user_mask) {
> + cpumask_copy(user_mask, in_mask);
> + } else {
> + return -ENOMEM;
> + }
> }
>
> ac = (struct affinity_context){
Not an expert on this code.
I'm only wondering if there is somehow, some way we could be breaking
user space by doing that.
--
Cheers
David / dhildenb
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity()
2025-10-20 20:13 ` David Hildenbrand
@ 2025-10-20 20:21 ` Waiman Long
0 siblings, 0 replies; 5+ messages in thread
From: Waiman Long @ 2025-10-20 20:21 UTC (permalink / raw)
To: David Hildenbrand, Ingo Molnar, Peter Zijlstra, Juri Lelli,
Vincent Guittot, Dietmar Eggemann, Steven Rostedt, Ben Segall,
Mel Gorman, Valentin Schneider, Jonathan Corbet
Cc: linux-kernel, linux-fsdevel, linux-doc, Andrew Morton,
Catalin Marinas, Nico Pache, Phil Auld, John Coleman
On 10/20/25 4:13 PM, David Hildenbrand wrote:
> On 23.09.25 19:54, Waiman Long wrote:
>> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
>> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
>> perserved even if the task is being moved to a different cpuset.
>> However, that affinity is also being inherited by any subsequently
>> created child processes which may not want or be aware of that affinity.
>
> So I assume setting the affinity to the full bitmap would then allow
> any child to essentially reset to the default, correct?
Yes, that is the point.
>
>>
>> One way to solve this problem is to provide a way to back off from
>> that user provided CPU affinity. This patch implements such a scheme
>> by using a full cpumask (a cpumask with all bits set) to signal the
>> clearing of the user cpumask to follow the default as allowed by
>> the current cpuset. In fact, with a full cpumask in user_cpus_ptr,
>> the task behavior should be the same as with a NULL user_cpus_ptr.
>> This patch just formalizes it without causing any incompatibility and
>> discard an otherwise useless cpumask.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> kernel/sched/syscalls.c | 20 ++++++++++++++------
>> 1 file changed, 14 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
>> index 77ae87f36e84..d68c7a4ee525 100644
>> --- a/kernel/sched/syscalls.c
>> +++ b/kernel/sched/syscalls.c
>> @@ -1229,14 +1229,22 @@ long sched_setaffinity(pid_t pid, const
>> struct cpumask *in_mask)
>> return retval;
>> /*
>> - * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
>> - * alloc_user_cpus_ptr() returns NULL.
>> + * If a full cpumask is passed in, clear user_cpus_ptr and reset
>> the
>> + * current cpu affinity to the default for the current cpuset.
>> */
>> - user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
>> - if (user_mask) {
>> - cpumask_copy(user_mask, in_mask);
>> + if (cpumask_full(in_mask)) {
>> + user_mask = NULL;
>> } else {
>> - return -ENOMEM;
>> + /*
>> + * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
>> + * alloc_user_cpus_ptr() returns NULL.
>> + */
>> + user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
>> + if (user_mask) {
>> + cpumask_copy(user_mask, in_mask);
>> + } else {
>> + return -ENOMEM;
>> + }
>> }
>> ac = (struct affinity_context){
>
> Not an expert on this code.
>
> I'm only wondering if there is somehow, some way we could be breaking
> user space by doing that.
>
I don't think so. Setting user_cpus_ptr to a full cpumask will make the
task strictly follow the cpumask restriction imposed by the current
cpuset as if user_cpus_ptr isn't set.
Cheers,
Longman
^ permalink raw reply [flat|nested] 5+ messages in thread