On 07/17/2014 10:39 AM, Christoph Lameter wrote: > On Thu, 17 Jul 2014, Pranith Kumar wrote: > >>> The use of atomic_t implies a remote write operation to a percpu area. >>> >>> atomic_t needs to be avoided. If data needs to be modified from multiple >>> cpus then it usually does not belong into a percpu area. >>> >> >> Yes, I think I made it pretty clear that remote accesses need to be avoided >> unless absolutely necessary. But, there will be scenarios where mostly local >> data will need to be have remote accesses. In such scenarios, isn't using >> atomic_t better? FYI, that is how RCU code currently works. It uses atomic_t in >> per cpu areas to ensure atomicity for remote accesses. > > 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. > >> 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. -- Pranith -- 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
