On Fri, Feb 03, 2023 at 11:40:40AM -0500, Waiman Long wrote: > Since commit 8f9ea86fdf99 ("sched: Always preserve the user > requested cpumask"), relax_compatible_cpus_allowed_ptr() is calling > __sched_setaffinity() unconditionally. This helps to expose a bug in > the current cpuset hotplug code where the cpumasks of the tasks in > the top cpuset are not updated at all when some CPUs become online or > offline. It is likely caused by the fact that some of the tasks in the > top cpuset, like percpu kthreads, cannot have their cpu affinity changed. > > One way to reproduce this as suggested by Peter is: > - boot machine > - offline all CPUs except one > - taskset -p ffffffff $$ > - online all CPUs > > Fix this by allowing cpuset_cpus_allowed() to return a wider mask that > includes offline CPUs for those tasks that are in the top cpuset. For > tasks not in the top cpuset, the old rule applies and only online CPUs > will be returned in the mask since hotplug events will update their > cpumasks accordingly. > > Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask") > Reported-by: Will Deacon <will@xxxxxxxxxx> > Originally-from: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx> So, this is the replacement for the first patch[1] Will posted, right? > void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) > { > unsigned long flags; > + struct cpuset *cs; > > spin_lock_irqsave(&callback_lock, flags); > - guarantee_online_cpus(tsk, pmask); > + rcu_read_lock(); > + > + cs = task_cs(tsk); > + if (cs != &top_cpuset) > + guarantee_online_cpus(tsk, pmask); > + /* > + * TODO: Tasks in the top cpuset won't get update to their cpumasks > + * when a hotplug online/offline event happens. So we include all > + * offline cpus in the allowed cpu list. > + */ > + if ((cs == &top_cpuset) || cpumask_empty(pmask)) { > + const struct cpumask *possible_mask = task_cpu_possible_mask(tsk); > + > + /* > + * We first exclude cpus allocated to partitions. If there is no > + * allowable online cpu left, we fall back to all possible cpus. > + */ > + cpumask_andnot(pmask, possible_mask, top_cpuset.subparts_cpus); and the differences are that * It's only applied to the root cgroup. * Cpus taken up by partitions are excluded. Is my understanding correct? > + if (!cpumask_intersects(pmask, cpu_online_mask)) > + cpumask_copy(pmask, possible_mask); > + } > + > + rcu_read_unlock(); > spin_unlock_irqrestore(&callback_lock, flags); So, I suppose you're suggesting applying this patch instead of the one Will Deacon posted[1] and we need Will's second patch[2] on top, right? [1] http://lkml.kernel.org/r/20230131221719.3176-3-will@xxxxxxxxxx [2] http://lkml.kernel.org/r/20230131221719.3176-3-will@xxxxxxxxxx Thanks. -- tejun