On 04/09/2019 04:40 PM, Joel Savitz wrote: > If a process is limited by taskset (i.e. cpuset) to only be allowed to > run on cpu N, and then cpu N is offlined via hotplug, the process will > be assigned the current value of its cpuset cgroup's effective_cpus field > in a call to do_set_cpus_allowed() in cpuset_cpus_allowed_fallback(). > This argument's value does not makes sense for this case, because > task_cs(tsk)->effective_cpus is modified by cpuset_hotplug_workfn() > to reflect the new value of cpu_active_mask after cpu N is removed from > the mask. While this may make sense for the cgroup affinity mask, it > does not make sense on a per-task basis, as a task that was previously > limited to only be run on cpu N will be limited to every cpu _except_ for > cpu N after it is offlined/onlined via hotplug. > > Pre-patch behavior: > > $ grep Cpus /proc/$$/status > Cpus_allowed: ff > Cpus_allowed_list: 0-7 > > $ taskset -p 4 $$ > pid 19202's current affinity mask: f > pid 19202's new affinity mask: 4 > > $ grep Cpus /proc/self/status > Cpus_allowed: 04 > Cpus_allowed_list: 2 > > # echo off > /sys/devices/system/cpu/cpu2/online > $ grep Cpus /proc/$$/status > Cpus_allowed: 0b > Cpus_allowed_list: 0-1,3 > > # echo on > /sys/devices/system/cpu/cpu2/online > $ grep Cpus /proc/$$/status > Cpus_allowed: 0b > Cpus_allowed_list: 0-1,3 > > On a patched system, the final grep produces the following > output instead: > > $ grep Cpus /proc/$$/status > Cpus_allowed: ff > Cpus_allowed_list: 0-7 > > This patch changes the above behavior by instead resetting the mask to > task_cs(tsk)->cpus_allowed by default, and cpu_possible mask in legacy > mode. > > This fallback mechanism is only triggered if _every_ other valid avenue > has been traveled, and it is the last resort before calling BUG(). > > Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx> > --- > kernel/cgroup/cpuset.c | 15 ++++++++++++++- > 1 file changed, 14 insertions(+), 1 deletion(-) > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > index 4834c4214e9c..6c9deb2cc687 100644 > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -3255,10 +3255,23 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) > spin_unlock_irqrestore(&callback_lock, flags); > } > > +/** > + * cpuset_cpus_allowed_fallback - final fallback before complete catastrophe. > + * @tsk: pointer to task_struct with which the scheduler is struggling > + * > + * Description: In the case that the scheduler cannot find an allowed cpu in > + * tsk->cpus_allowed, we fall back to task_cs(tsk)->cpus_allowed. In legacy > + * mode however, this value is the same as task_cs(tsk)->effective_cpus, > + * which will not contain a sane cpumask during cases such as cpu hotplugging. > + * This is the absolute last resort for the scheduler and it is only used if > + * _every_ other avenue has been traveled. > + **/ > + > void cpuset_cpus_allowed_fallback(struct task_struct *tsk) > { > rcu_read_lock(); > - do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus); > + do_set_cpus_allowed(tsk, is_in_v2_mode() ? > + task_cs(tsk)->cpus_allowed : cpu_possible_mask); > rcu_read_unlock(); > > /* Acked-by: Waiman Long <longman@xxxxxxxxxx>