On Mon, Feb 17, 2020 at 10:31:24AM +0100, Michal Hocko wrote: >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. I am fine with this suggestion. Let me prepare v3. >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me
