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Thanks for taking time looking into this. >> In current upstream, lazy is a preemption mode with preempt_enable being a preemptible point, >> >> 1. If PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n, then PREEMPT_RCU=n. That seems wrong. It is supposed to >> do the preemption checks, since preempt_count is actual count with lazy preemption. > > Sorry, but no, not wrong at all. This is the way that it is supposed > to be. In this configuration, RCU readers explicitly disable preemption. > This gets us minimal RCU overhead on architectures such as x86 and arm64 > that have prohibited CONFIG_PREEMPT_NONE and CONFIG_PREEMPT_VOLUNTARY. This is after 7.0 only right? Before that one could choose even on x86 and arm64 IIUC. After 7.0, even on powerpc, one cannot choose them anymore. > Not zero overhead, but minimal overhead given the constraints imposed by > CONFIG_PREEMPT_LAZY. This also prevents preemption of RCU readers, which > is extremely important on datacenter servers that have good and sufficient > quantities of memory, but which are not well-endowed with memory. > >> 2. PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n and PREEMPT_LAZY=y and PREEMPT_DYNAMIC=y are supposed >> to be similar w.r.t to performance. > > RCU never made that promise. ;-) > > With PREMPT_DYNAMIC=y, RCU has no choice but to be preemptible, at > least unless and until someone decides to make RCU's preemptibility be > changeable at runtime (which I do not recommend). And we really do need > non-preemptible RCU for server workloads. most of the distros i see set PREMPT_DYNAMIC=y. I checked in SLES, RHEL. (I could be wrong here) > > So it is not just that RCU never made that promise, it is that RCU cannot > reasonably make that promise. > >> Promise of PREEMPT_DYNAMIC was the ability to >> switch at runtime based on the user workload and his/her preference. >> But with PREEMPT_LAZY=y and PREEMPT_DYNAMIC=y, there is additional cost due to PREEMPT_RCU. >> lazy being the middle ground w.r.t for decent performance, either PREEMPT_RCU should either kick in >> for both cases or bail out for both. > > Absolutely not!!! Please see above. > >> Before 7.0 >> 3. the same concerns would be true to PREEMPT_VOLUNTARY and PREEMPT_NONE with PREEMPT_DYNAMIC=y. >> Those model never wanted a preemption, but PREEMPT_RCU could have forced a need_resched to be >> set. Which is also likely wrong. No? > > You lost me on this one. Please give me an explicit sequence of events > that would cause things to go wrong in this case. You are right. PREEMPT_RCU may set need_resched. But even then on none/voluntary schedule will not be called since static keys it cannot.