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

[Xuewen Yan]

Hi Saket

On Fri, Jun 7, 2024 at 6:02 PM Saket Kumar Bhaskar <skb99@xxxxxxxxxxxxx> wrote:
>
> 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@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
> >
>
> 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?

Whether the thread you are testing belongs to top-cpuset? If so, I
think is normal.
And you can see the patch: 3fb906e7fabbb (group/cpuset: Don't filter
offline CPUs in cpuset_cpus_allowed() for top cpuset tasks).

BR
---
xuewen
>
> Thanks and regards,
> Saket Kumar Bhaskar