On Sat 15-02-20 08:37:53, Wei Yang wrote: > On Fri, Feb 14, 2020 at 09:51:13AM +0100, Michal Hocko wrote: > >On Fri 14-02-20 15:33:20, Wei Yang wrote: > >> When onlining a cpu, kswapd_cpu_online() is called to adjust kswapd cpu > >> affinity. > >> > >> Current routine does like this: > >> > >> a) Iterate all the numa node > >> b) Adjust cpu affinity when node has an online cpu > >> > >> For a) this is not necessary, since the particular online cpu belongs to > >> a particular numa node. So it is not necessary to iterate on every nodes > >> on the system. This new onlined cpu just affect kswapd cpu affinity of > >> this particular node. > >> > >> For b) several cpumask operation is used to check whether the node has > >> an online CPU. Since at this point we are sure one of our CPU onlined, > >> we can set the cpu affinity directly to current cpumask_of_node(). > >> > >> This patch simplifies the logic by set cpu affinity of the affected > >> kswapd. > > > >How have you tested this patch? > > > > I online one cpu and confirm the "cpu" is the one we just onlined. > > If my understanding is correct, this is the expected behavior. > > >Also this is an old code and quite convoluted but does it still work as > >inteded? I mean, I do not see any cpu offline callback to reduce the > >cpu mask as all the CPUs for the given node go offline? Wouldn't the > > You are right, I didn't see the counterpart for cpu offline. This is the > question I want to ask. Seems we didn't handle it at the very beginning. > > >scheduler simply go and fallback to no affinity if that happens? > >In other words what is the value of kswapd_cpu_online in the first > >place? > > Some cases may this function be useful. > > If we have a memory node which doesn't have any online cpu, the default > cpumask is not set. After one of the cpu online, we want to change cpu > affinity. > > Or we want to add more cpu to the system, we could allow kswapd use more cpu > resources. Otherwise, kswapd would be limited to those original cpus. OK, so the usecase is when a NUMA node gains a new CPU which wasn't there at the time when the node got onlined. Is this a scenario we really do care about? While not completely impossible I haven't seen a system which would allow such a runtime configurability. Maybe it would be simply easier to drop the callback for now until we have a real world usecase to support it and have it documented. -- Michal Hocko SUSE Labs
