On 07/17/2014 10:55 AM, Christoph Lameter wrote: > 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. I can mention that IPI is preferable. What is that you don't want mentioned? atomic_t? > >>>> 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. > Yes, -usually- that is the case. But we are talking about the extreme rare event where we need to update some remote CPU`s per-cpu data without waking it up from sleep/idle. How do you suggest we handle this? I don't think suggesting not to use per-cpu areas because of this is a good idea, since we lose a lot of performance in the most common cases. Thoughts? -- 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
