On 11.05.22 11:34, Daniel P. Berrangé wrote: > On Wed, May 11, 2022 at 11:31:23AM +0200, David Hildenbrand wrote: >>>> Long story short, management application has no way of learning >>>> TIDs of allocator threads so it can't make them run NUMA aware. >>> >>> This feels like the key issue. The preallocation threads are >>> invisible to libvirt, regardless of whether we're doing coldplug >>> or hotplug of memory-backends. Indeed the threads are invisible >>> to all of QEMU, except the memory backend code. >>> >>> Conceptually we need 1 or more explicit worker threads, that we >>> can assign CPU affinity to, and then QEMU can place jobs on them. >>> I/O threads serve this role, but limited to blockdev work. We >>> need a generalization of I/O threads, for arbitrary jobs that >>> QEMU might want to farm out to specific numa nodes. >> >> At least the "-object iothread" thingy can already be used for actions >> outside of blockdev. virtio-balloon uses one for free page hinting. > > Ah that's good to know, so my idea probably isn't so much work as > I thought it might be.
Looking into the details, iothreads are the wrong tool to use here. I can imagine use cases where you'd want to perform preallcoation * Only one some specific CPUs of a NUMA node (especially, not ones where we pinned VCPUs) * On CPUs that are on a different NUMA node then the actual backend memory ... just thinking about all of the CPU-less nodes for PMEM and CXL that we're starting to see. So ideally, we'd let the user configure either * Set of physical CPUs to use (low hanging fruit) * Set of NUMA nodes to use (harder) and allow libvirt to easily configure it similarly by pinning threads. As CPU affinity is inherited when creating a new thread, so here is one IMHO reasonable simple thing to get the job done and allow for such flexibility. Introduce a "thread-context" user-creatable object that is used to configure CPU affinity. Internally, thread-context creates exactly one thread called "TC $id". That thread serves no purpose besides spawning new threads that inherit the affinity. Internal users (like preallocation, but we could reuse the same concept for other non-io threads, such as userfaultfd, and some other potential non-io thread users) simply use that thread context to spawn new threads. Spawned threads get called "TC $id/$threadname", whereby $threadname is the ordinary name supplied by the internal user. This could be used to identify long-running threads if needed in the future. It's worth nothing that this is not a thread pool. a) Ordinary cmdline usage: -object thread-context,id="tc1",cpus=0-9,cpus=12 QEMU tries setting the CPU affinity and fails if that's impossible. -object memory-backend-file,...,prealloc=on,prealloc-threads=16,prealloc-thread-context=tc1 When a context is set, preallcoation code will use the thread-context to spawn threads. b) Libvirt, ..., usage: -object thread-context,id="tc1" Then, libvirt identifies and sets the affinity for the "TC tc1" thread. -object memory-backend-file,...,prealloc=on,prealloc-threads=16,prealloc-thread-context=tc1 thread-context can be reused for successive preallcoation etc, obviously. -- Thanks, David / dhildenb