On Wed, 30 Nov 2022 18:41:16 +0000 bugzi...@dpdk.org wrote: > https://bugs.dpdk.org/show_bug.cgi?id=1137 > > Bug ID: 1137 > Summary: CPU affinity set incorrectly when lcore_id 0 is not > the master-lcore > Product: DPDK > Version: 22.11 > Hardware: All > OS: Linux > Status: UNCONFIRMED > Severity: normal > Priority: Normal > Component: core > Assignee: dev@dpdk.org > Reporter: ltr...@cisco.com > Target Milestone: --- > > Created attachment 233 > --> https://bugs.dpdk.org/attachment.cgi?id=233&action=edit > Logs showing incorrect CPU set assignment > > When a range of CPUs are used (e.g. 0-3), and the master-lcore is set to > non-zero, the CPU affinity for lcore-id 0 is set incorrectly, due to its > cpuset > being overwritten by the control-thread creation. > > CPU arguments passed are '-c f --master-lcore 3', to indicate that CPUs 0-3 > should be used, with the master on CPU 3. In particular, DPDK itself is > initialized from CPU 3. > > When the control threads (eal-intr-thread, rte_mp_handle) are created, they > are > initialized from CPU3 - so inherit the cpuset containing just this CPU. When > calling __rte_thread_init(), ctrl_thread_init() passes the result of > rte_lcore_id() - but this is not yet initialized for this thread - so is set > to > 0. > > This means that internally, the lcore_id for the control-thread is set to 0 - > and > in particular, the call to thread_update_affinity() overwrites the cpuset for > lcore_id=0 with the cpuset of CPU3: > > memmove(&lcore_config[lcore_id].cpuset, cpusetp, > sizeof(rte_cpuset_t)); > > This all occurs before the main __rte_thread_init() call for each Slave thread > - so that the slave thread associated with lcore_id, which should be running > on > CPU0, instead has its affinity incorrectly set to CPU3. > > RTE logs are attached showing this behavior (and including some additional > logs > added locally to print the lcore-id and cpusets being passed). > > The fix for this should be to make ctrl_thread_init() more similar to > rte_thread_register(), so that it calls eal_lcore_non_eal_allocate() to assign > an lcore-id, then passes this to __rte_thread_init(). I have tested a fix for > this locally to confirm. >
Side note: using CPU 0 with DPDK is not recommended for any real application. It is impossible to fully isolate CPU 0 and therefore you will get poor performance and mystery latency spikes.
