Hi, I observed that aarch64 boot time increases considerably with rt (PREEMPT_RT_FULL=y). This is observable also in latest 4.19 rt (4.19.127-rt55). Following is a fragment from kernel log (with rt) which shows that bringing up 4 CPUs takes about 210 ms: [ 0.120951] smp: Bringing up secondary CPUs ... [ 0.184860] Detected VIPT I-cache on CPU1 [ 0.187344] CPU1: Booted secondary processor 0x0000000001 [0x410fd034] [ 0.262931] Detected VIPT I-cache on CPU2 [ 0.263390] CPU2: Booted secondary processor 0x0000000002 [0x410fd034] [ 0.329296] Detected VIPT I-cache on CPU3 [ 0.329872] CPU3: Booted secondary processor 0x0000000003 [0x410fd034] [ 0.331557] smp: Brought up 1 node, 4 CPUs Note that these times are from qemu, however the problem is also noticeable on real hardware. I traced the code and found that the slow down is caused in workqueue_prepare_cpu(). This function creates the worker threads. On real hardware, it takes about 8 ms to create each worker thread. Before digging into it in more detail, I would like to hear your opinion. Steven, what ftrace command line parameters do you recommend to use in this case? Thank you, Itai Handler
