On Wed, May 26, 2021 at 06:02:06PM +0100, Will Deacon wrote: > On Wed, May 26, 2021 at 06:30:08PM +0200, Peter Zijlstra wrote: > > On Tue, May 25, 2021 at 04:14:23PM +0100, Will Deacon wrote: > > > @@ -2426,20 +2421,166 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, > > > > > > __do_set_cpus_allowed(p, new_mask, flags); > > > > > > - return affine_move_task(rq, p, &rf, dest_cpu, flags); > > > + if (flags & SCA_USER) > > > + release_user_cpus_ptr(p); > > > + > > > + return affine_move_task(rq, p, rf, dest_cpu, flags); > > > > > > out: > > > - task_rq_unlock(rq, p, &rf); > > > + task_rq_unlock(rq, p, rf); > > > > > > return ret; > > > } > > > > So sys_sched_setaffinity() releases the user_cpus_ptr thingy ?! How does > > that work? > > Right, I think if the task explicitly changes its affinity then it makes > sense to forget about what it had before. It then behaves very similar to > CPU hotplug, which is the analogy I've been trying to follow: if you call > sched_setaffinity() with a mask containing offline CPUs then those CPUs > are not added back to the affinity mask when they are onlined. Oh right, crap semantics all the way down :/ I always forget how horrible they are. You're right though; this is consistent with the current mess.
