----- On Jan 31, 2022, at 4:19 PM, Florian Weimer fw@xxxxxxxxxxxxx wrote: > * Mathieu Desnoyers: > >> Adding the NUMA node id to struct rseq is a straightforward thing to do, >> and a good way to figure out if anything in the user-space ecosystem >> prevents extending struct rseq. >> >> This NUMA node id field allows memory allocators such as tcmalloc to >> take advantage of fast access to the current NUMA node id to perform >> NUMA-aware memory allocation. >> >> It is also useful for implementing NUMA-aware user-space mutexes. > > It can be used to implement getcpu purely in userspace, too. I had > plan to hack this together with a node ID cache in TLS, which should > offer pretty much the same functionality (except for weird CPU > topology changes which alter the node ID of a previously used CPU). I suspect that any approach based on a user-space cache will break with respect to CRIU. That is one big advantage of using the rseq thread area for this. > > However, I do not understand the need for two fields here. Why isn't > one enough? As stated in my self-reply, I don't think those two fields are needed after all. > > One field would also avoid the need to mess with rseq_cpu_id_state, > maintaining API compatibility. True. However considering that we plan to remove the buggy "rseq_cs.ptr" fields from the API, that rseq.h UAPI compatibility does not seem to be very much relevant. But still, it's better if we can avoid breaking API, agreed. And the "node_id_start" does not appear to be needed after all. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
