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Date: Fri, 7 Jun 2024 15:32:36 +0530
From: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
To: Xuewen Yan <xuewen.yan@...soc.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
vincent.guittot@...aro.org, longman@...hat.com,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
ke.wang@...soc.com, xuewen.yan94@...il.com,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched: Clear user_cpus_ptr only when no intersection
with the new mask
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@...hat.com>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.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
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