Re: [RFC PATCH] sched: Clear user_cpus_ptr only when no intersection with the new mask

[Xuewen Yan]

Friendly ping...

On Tue, May 7, 2024 at 3:23 PM Xuewen Yan <xuewen.yan@xxxxxxxxxx> wrote:
>
> The commit 851a723e45d1c("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> would cause that online/offline cpu will produce different results
> for the !top-cpuset task.
> For example:
>
> If the task was running, then offline task's cpus, would lead to clear
> its user-mask.
>
> unisoc:/ # while true; do sleep 600; done&
> [1] 6786
> unisoc:/ # echo 6786 > /dev/cpuset/top-app/tasks
> unisoc:/ # cat /dev/cpuset/top-app/cpus
> 0-7
> unisoc:/ # cat /proc/6786/status | grep Cpus
> Cpus_allowed:   ff
> Cpus_allowed_list:      0-7
>
> unisoc:/ # taskset -p c0 6786
> pid 6786's current affinity mask: ff
> pid 6786's new affinity mask: c0
> unisoc:/ # cat /proc/6786/status | grep Cpus
> Cpus_allowed:   c0
> Cpus_allowed_list:      6-7
>
> After offline the cpu6 and cpu7, the user-mask would be cleared:
>
> unisoc:/ # echo 0 > /sys/devices/system/cpu/cpu7/online
> unisoc:/ # cat /proc/6786/status | grep Cpus
> Cpus_allowed:   40
> Cpus_allowed_list:      6
> ums9621_1h10:/ # echo 0 > /sys/devices/system/cpu/cpu6/online
> ums9621_1h10:/ # cat /proc/6786/status | grep Cpus
> Cpus_allowed:   3f
> Cpus_allowed_list:      0-5
>
> When online the cpu6/7, the user-mask can not bring back:
>
> unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu6/online
> unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu7/online
> unisoc:/ # cat /proc/6786/status | grep Cpus
> Cpus_allowed:   ff
> Cpus_allowed_list:      0-6
>
> However, if we offline the cpu when the task is sleeping, at this
> time, because would not call the fallback_cpu(), its user-mask will
> not be cleared.
>
> unisoc:/ # while true; do sleep 600; done&
> [1] 5990
> unisoc:/ # echo 5990 > /dev/cpuset/top-app/tasks
> unisoc:/ # cat /proc/5990/status | grep Cpus
> Cpus_allowed:   ff
> Cpus_allowed_list:      0-7
>
> unisoc:/ # taskset -p c0 5990
> pid 5990's current affinity mask: ff
> pid 5990's new affinity mask: c0
> unisoc:/ # cat /proc/5990/status | grep Cpus
> Cpus_allowed:   c0
> Cpus_allowed_list:      6-7
>
> unisoc:/ # echo 0 > /sys/devices/system/cpu/cpu6/online
> unisoc:/ # cat /proc/5990/status | grep Cpus
> Cpus_allowed:   80
> Cpus_allowed_list:      7
> unisoc:/ # echo 0 > /sys/devices/system/cpu/cpu7/online
> unisoc:/ # cat /proc/5990/status | grep Cpus
> Cpus_allowed:   3f
> Cpus_allowed_list:      0-5
>
> After 10 minutes, it was waked up, it can also keep its user-mask:
> ums9621_1h10:/ # cat /proc/5990/status | grep Cpus
> Cpus_allowed:   3f
> Cpus_allowed_list:      0-5
>
> And when online the cpu6/7,the user-mask could bring back.
> unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu6/online
> unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu7/online
> unisoc:/ # cat /proc/6786/status | grep Cpus
> Cpus_allowed:   c0
> Cpus_allowed_list:      6-7
>
> Indeed, there is no need to clear the user_cpus_ptr if there is an
> intersection between user_cpus_ptr and new_mask.
> So add the judgement of whether there is an intersection between them.
> Clear user_cpus_ptr only when no intersection with the new mask.
> In this way, the above problems can also be solved.
>
> Suggested-by: Waiman Long <longman@xxxxxxxxxx>
> Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> ---
> previous discussion:
>  https://lore.kernel.org/all/e402d623-1875-47a2-9db3-8299a54502ef@xxxxxxxxxx/
> ---
>  kernel/sched/core.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7019a40457a6..bbb8e88949f4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2796,21 +2796,24 @@ __do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
>  }
>
>  /*
> - * Used for kthread_bind() and select_fallback_rq(), in both cases the user
> - * affinity (if any) should be destroyed too.
> + * Used for kthread_bind() and select_fallback_rq().
> + * Destroy user affinity if no intersection with the new_mask.
>   */
>  void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>  {
>         struct affinity_context ac = {
>                 .new_mask  = new_mask,
>                 .user_mask = NULL,
> -               .flags     = SCA_USER,  /* clear the user requested mask */
> +               .flags     = 0,
>         };
>         union cpumask_rcuhead {
>                 cpumask_t cpumask;
>                 struct rcu_head rcu;
>         };
>
> +       if (p->user_cpus_ptr && !cpumask_intersects(p->user_cpus_ptr, new_mask))
> +               ac.flags = SCA_USER;    /* clear the user requested mask */
> +
>         __do_set_cpus_allowed(p, &ac);
>
>         /*
> --
> 2.25.1
>
>