On Thu, Dec 03, 2020 at 05:14:31PM +0000, Alexey Klimov wrote: > When a CPU offlined and onlined via device_offline() and device_online() > the userspace gets uevent notification. If, after receiving uevent, > userspace executes sched_setaffinity() on some task trying to move it > to a recently onlined CPU, then it will fail with -EINVAL. Userspace needs > to wait around 5..30 ms before sched_setaffinity() will succeed for > recently onlined CPU after receiving uevent. Right. > Unfortunately, the execution time of > echo 1 > /sys/devices/system/cpu/cpuX/online roughly doubled with this > change (on my test machine). Nobody cares, it's hotplug, it's supposed to be slow :-) That is, we fundamentally shift the work _to_ the hotplug path, so as to keep everybody else fast. > The nature of this bug is also described here (with different consequences): > https://lore.kernel.org/lkml/20200211141554.24181-1-qais.yousef@xxxxxxx/ Yeah, pesky deadlocks.. someone was going to try again. > kernel/cpu.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 6ff2578ecf17..f39a27a7f24b 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -15,6 +15,7 @@ > #include <linux/sched/smt.h> > #include <linux/unistd.h> > #include <linux/cpu.h> > +#include <linux/cpuset.h> > #include <linux/oom.h> > #include <linux/rcupdate.h> > #include <linux/export.h> > @@ -1275,6 +1276,8 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target) > } > > err = _cpu_up(cpu, 0, target); > + if (!err) > + cpuset_wait_for_hotplug(); > out: > cpu_maps_update_done(); > return err; My only consideration is if doing that flush under cpu_add_remove_lock() is wise.
