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

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

On Tue, May 07, 2024 at 03:22:42PM +0800, Xuewen Yan 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@redhat.com>
> Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
> ---
> previous discussion:
>  https://lore.kernel.org/all/e402d623-1875-47a2-9db3-8299a54502ef@redhat.com/
> ---
>  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
> 

Hi Xuewen,

I have a query here:

1. From the example where task is running, " while true; do sleep 600; done& "
   is being used, which is a sleeping task. How running task is emulated here?

2. Tried without patch (6.9.0-rc4) on a 64 CPUs system. Results are:
   
   For a task that is running:

   # stress-ng -l 100 --cpu 1
   stress-ng: info:  [2307] defaulting to a 86400 second (1 day, 0.00 secs) run per stressor
   stress-ng: info:  [2307] dispatching hogs: 1 cpu
   
   #taskset -p c0 2308
   pid 2308's current affinity mask: ffffffffffffffff
   pid 2308's new affinity mask: c0
   
   # cat /proc/2308/status |grep Cpus
   Cpus_allowed:	00000000,000000c0
   Cpus_allowed_list:	6-7
   
   #chcpu -d 6,7
   CPU 6 disabled
   CPU 7 disabled
   
   After disabling CPUs 6 and 7:

   # cat /proc/2308/status |grep Cpus
   Cpus_allowed:	ffffffff,ffffffff
   Cpus_allowed_list:	0-63

   After enabling CPUs 6 and 7:

   ## chcpu -e 6,7
   CPU 6 enabled
   CPU 7 enabled
   
   # cat /proc/2308/status |grep Cpus
   Cpus_allowed:	ffffffff,ffffffff
   Cpus_allowed_list:	0-63

   From the above output, after disabling CPUs 6 and 7, all the CPUs in the
   system are displayed rather than showing only remaining online CPUs(as 
   shown in above example).

   For a task that is sleeping:

   # while true; do sleep 60; done&
   [1] 2541
   
   # taskset -p c0 2541
   pid 2541's current affinity mask: ffffffffffffffff
   pid 2541's new affinity mask: c0
   
   # cat /proc/2541/status |grep Cpus
   Cpus_allowed:	00000000,000000c0
   Cpus_allowed_list:	6-7
 
   After disabling CPUs 6 and 7:
  
   # chcpu -d 6,7
   CPU 6 disabled
   CPU 7 disabled

   # cat /proc/2541/status |grep Cpus
   Cpus_allowed:	00000000,000000c0
   Cpus_allowed_list:	6-7
   
   After 1 minute:

   # cat /proc/2541/status |grep Cpus
   Cpus_allowed:	ffffffff,ffffffff
   Cpus_allowed_list:	0-63
   
   # chcpu -e 6,7
   CPU 6 enabled
   CPU 7 enabled
   
   # cat /proc/2541/status |grep Cpus
   Cpus_allowed:	ffffffff,ffffffff
   Cpus_allowed_list:	0-63

   From the above output, after disabling CPUs 6 and 7, it waked up after 
   1 minute and Cpus_allowed_list got changed to 0-63 (which is contrary 
   to the above example).

   So, there is some deviation in behaviour seen without the patch, 
   than reported or am I missing something?

Thanks and regards,
Saket Kumar Bhaskar