Re: [PATCH v5 08/15] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Dec 8, 2020 at 5:29 AM Will Deacon <will@kernel.org> wrote:
>
> Asymmetric systems may not offer the same level of userspace ISA support
> across all CPUs, meaning that some applications cannot be executed by
> some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
> not feature support for 32-bit applications on both clusters.
>
> Modify guarantee_online_cpus() to take task_cpu_possible_mask() into
> account when trying to find a suitable set of online CPUs for a given
> task. This will avoid passing an invalid mask to set_cpus_allowed_ptr()
> during ->attach() and will subsequently allow the cpuset hierarchy to be
> taken into account when forcefully overriding the affinity mask for a
> task which requires migration to a compatible CPU.
>
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  include/linux/cpuset.h |  3 ++-
>  kernel/cgroup/cpuset.c | 33 +++++++++++++++++++--------------
>  2 files changed, 21 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 04c20de66afc..414a8e694413 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -15,6 +15,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/nodemask.h>
>  #include <linux/mm.h>
> +#include <linux/mmu_context.h>
>  #include <linux/jump_label.h>
>
>  #ifdef CONFIG_CPUSETS
> @@ -184,7 +185,7 @@ static inline void cpuset_read_unlock(void) { }
>  static inline void cpuset_cpus_allowed(struct task_struct *p,
>                                        struct cpumask *mask)
>  {
> -       cpumask_copy(mask, cpu_possible_mask);
> +       cpumask_copy(mask, task_cpu_possible_mask(p));
>  }
>
>  static inline void cpuset_cpus_allowed_fallback(struct task_struct *p)
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index e970737c3ed2..d30febf1f69f 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -372,18 +372,26 @@ static inline bool is_in_v2_mode(void)
>  }
>
>  /*
> - * Return in pmask the portion of a cpusets's cpus_allowed that
> - * are online.  If none are online, walk up the cpuset hierarchy
> - * until we find one that does have some online cpus.
> + * Return in pmask the portion of a task's cpusets's cpus_allowed that
> + * are online and are capable of running the task.  If none are found,
> + * walk up the cpuset hierarchy until we find one that does have some
> + * appropriate cpus.
>   *
>   * One way or another, we guarantee to return some non-empty subset
>   * of cpu_online_mask.
>   *
>   * Call with callback_lock or cpuset_mutex held.
>   */
> -static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
> +static void guarantee_online_cpus(struct task_struct *tsk,
> +                                 struct cpumask *pmask)
>  {
> -       while (!cpumask_intersects(cs->effective_cpus, cpu_online_mask)) {
> +       struct cpuset *cs = task_cs(tsk);
> +       const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
> +
> +       if (WARN_ON(!cpumask_and(pmask, possible_mask, cpu_online_mask)))

IIUC, this represents the case when there is no online CPU that can
run this task. In this situation guarantee_online_cpus() will return
an online CPU which can't run the task (because we ignore
possible_mask). I don't think this can be considered a valid fallback
path. However I think patch [13/15] ensures that we never end up in
this situation by disallowing to offline the last 32-bit capable CPU.
If that's true then maybe the patches can be reordered so that [13/15]
comes before this one and this condition can be treated as a bug here?


> +               cpumask_copy(pmask, cpu_online_mask);
> +
> +       while (!cpumask_intersects(cs->effective_cpus, pmask)) {
>                 cs = parent_cs(cs);
>                 if (unlikely(!cs)) {
>                         /*
> @@ -393,11 +401,10 @@ static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
>                          * cpuset's effective_cpus is on its way to be
>                          * identical to cpu_online_mask.
>                          */
> -                       cpumask_copy(pmask, cpu_online_mask);
>                         return;
>                 }
>         }
> -       cpumask_and(pmask, cs->effective_cpus, cpu_online_mask);
> +       cpumask_and(pmask, pmask, cs->effective_cpus);
>  }
>
>  /*
> @@ -2176,15 +2183,13 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>
>         percpu_down_write(&cpuset_rwsem);
>
> -       /* prepare for attach */
> -       if (cs == &top_cpuset)
> -               cpumask_copy(cpus_attach, cpu_possible_mask);
> -       else
> -               guarantee_online_cpus(cs, cpus_attach);
> -
>         guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
>
>         cgroup_taskset_for_each(task, css, tset) {
> +               if (cs != &top_cpuset)
> +                       guarantee_online_cpus(task, cpus_attach);
> +               else
> +                       cpumask_copy(cpus_attach, task_cpu_possible_mask(task));
>                 /*
>                  * can_attach beforehand should guarantee that this doesn't
>                  * fail.  TODO: have a better way to handle failure here
> @@ -3280,7 +3285,7 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>
>         spin_lock_irqsave(&callback_lock, flags);
>         rcu_read_lock();
> -       guarantee_online_cpus(task_cs(tsk), pmask);
> +       guarantee_online_cpus(tsk, pmask);
>         rcu_read_unlock();
>         spin_unlock_irqrestore(&callback_lock, flags);
>  }
> --
> 2.29.2.576.ga3fc446d84-goog
>