Messages in this thread | | | Date | Wed, 4 Nov 2020 15:45:21 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 05/16] rcu: De-offloading CB kthread |
| |
On Wed, Nov 04, 2020 at 10:42:09PM +0800, Boqun Feng wrote: > On Wed, Nov 04, 2020 at 03:31:35PM +0100, Frederic Weisbecker wrote: > [...] > > > > > > > + rcu_segcblist_offload(cblist, false); > > > > + raw_spin_unlock_rcu_node(rnp); > > > > + > > > > + if (rdp->nocb_cb_sleep) { > > > > + rdp->nocb_cb_sleep = false; > > > > + wake_cb = true; > > > > + } > > > > + rcu_nocb_unlock_irqrestore(rdp, flags); > > > > + > > > > + if (wake_cb) > > > > + swake_up_one(&rdp->nocb_cb_wq); > > > > + > > > > + swait_event_exclusive(rdp->nocb_state_wq, > > > > + !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)); > > > > + > > > > + return 0; > > > > +} > > > > + > > > > +static long rcu_nocb_rdp_deoffload(void *arg) > > > > +{ > > > > + struct rcu_data *rdp = arg; > > > > + > > > > + WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id()); > > > > > > I think this warning can actually happen, if I understand how workqueue > > > works correctly. Consider that the corresponding cpu gets offlined right > > > after the rcu_nocb_cpu_deoffloaed(), and the workqueue of that cpu > > > becomes unbound, and IIUC, workqueues don't do migration during > > > cpu-offlining, which means the worker can be scheduled to other CPUs, > > > and the work gets executed on another cpu. Am I missing something here?. > > > > We are holding cpus_read_lock() in rcu_nocb_cpu_offload(), this should > > prevent from that. > > > > But what if the work doesn't get executed until we cpus_read_unlock() > and someone offlines that CPU?
work_on_cpu() waits for completion before returning.
> Regards, > Boqun > > > Thanks!
| |