On 8/7/24 18:02, Frederic Weisbecker wrote: > Kthreads attached to a preferred NUMA node for their task structure > allocation can also be assumed to run preferrably within that same node. > > A more precise affinity is usually notified by calling > kthread_create_on_cpu() or kthread_bind[_mask]() before the first wakeup. > > For the others, a default affinity to the node is desired and sometimes > implemented with more or less success when it comes to deal with hotplug > events and nohz_full / CPU Isolation interactions: > > - kcompactd is affine to its node and handles hotplug but not CPU Isolation > - kswapd is affine to its node and ignores hotplug and CPU Isolation > - A bunch of drivers create their kthreads on a specific node and > don't take care about affining further. > > Handle that default node affinity preference at the generic level > instead, provided a kthread is created on an actual node and doesn't > apply any specific affinity such as a given CPU or a custom cpumask to > bind to before its first wake-up. > > This generic handling is aware of CPU hotplug events and CPU isolation > such that: > > * When a housekeeping CPU goes up and is part of the node of a given > kthread, it is added to its applied affinity set (and > possibly the default last resort online housekeeping set is removed > from the set). > > * When a housekeeping CPU goes down while it was part of the node of a > kthread, it is removed from the kthread's applied > affinity. The last resort is to affine the kthread to all online > housekeeping CPUs. > > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
