On Thu, 17 Jul 2014, Pranith Kumar wrote: > > The RCU code has .... ummmm... some issues with percpu usage and should > > not be taken as a good example. If you look at the RCU code it looks > > horrible with numerous barriers around remote percpu read/wrirte > > accesses and one wonders if that code is actually ok. > > Well, it is running in all our kernels with not many reported issues, isn't it ;) > And yes, that is one of the extra-ordinary situations where we use per-cpu data. > Once you've extracted a pointer to the per-cpu area -and- ensure that concurrent > accesses do not happen(or happen with enough guarantees using barriers), what is > the case against remote accesses? I am asking from a correctness and a > performance point of view, not style/aesthetics. Could be working but I do not want it to be mentioned in the documentation given the problems it causes. IPI is preferable. > >> If data needs to be modified from multiple cpus only very rarely, doesn't it > >> make sense to use per-cpu areas? > > > > I would suggest that this should not occur. You can always "modify" remote > > percpu areas by generating an IPI on that cpu to make that processor > > update its own per cpu data. > > > > The case against doing that is not to wake up CPUs which are in idle/sleep > states. I think mentioning it here that remote accesses are strongly discouraged > with a reasonable explanation of the implications should be enough. There might > always be rare situations where remote accesses might be necessary. Remote percpu updates are extremely rare events. If the cpu is idle/asleep then usually no updates are needed because no activity is occurring on that cpu. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
