On 8/30/2022 12:22 PM, Frederic Weisbecker wrote: > On Tue, Aug 30, 2022 at 06:03:16PM +0200, Frederic Weisbecker wrote: >> On Tue, Aug 30, 2022 at 04:43:43AM -0700, Paul E. McKenney wrote: >>> On Tue, Aug 30, 2022 at 12:53:24PM +0200, Frederic Weisbecker wrote: >>>> On Mon, Aug 29, 2022 at 01:42:02PM -0700, Paul E. McKenney wrote: >>>>> On Mon, Aug 29, 2022 at 04:36:40PM -0400, Joel Fernandes wrote: >>>>>> On Mon, Aug 29, 2022 at 3:46 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: >>>>>>> On Mon, Aug 29, 2022 at 12:45:40PM -0400, Joel Fernandes wrote: >>>>>>>> On 8/29/2022 9:40 AM, Frederic Weisbecker wrote: >>>>> >>>>> [ . . . ] >>>>> >>>>>>>>> 2) NOCB implies performance issues. >>>>>>>> >>>>>>>> Which kinds of? There is slightly worse boot times, but I'm guessing that's do >>>>>>>> with the extra scheduling overhead of the extra threads which is usually not a >>>>>>>> problem except that RCU is used in the critical path of boot up (on ChromeOS). >>>>>>> >>>>>>> I never measured it myself but executing callbacks on another CPUs, with >>>>>>> context switches and locking can only involve significant performance issues if callbacks >>>>>>> are frequent. So it's a tradeoff between power and performance. >>>>>> >>>>>> In my testing of benchmarks on real systems with 8-16 CPUs, the >>>>>> performance hit is down in the noise. It is possible though that maybe >>>>>> one can write a non-realistic synthetic test to force the performance >>>>>> issues, but I've not seen it in the real world. Maybe on >>>>>> networking-heavy servers with lots of cores, you'll see it but their >>>>>> batteries if any would be pretty big :-). >>>>> >>>>> To Frederic's point, if you have enough servers, even a 1% decrease in >>>>> power consumption is a very big deal. ;-) >>>> >>>> The world has enough servers, for that matters ;-) >>> >>> True enough! Now you need only demonstrate that call_rcu_lazy() for >>> !rcu_nocbs servers would actually deliver that 1%. ;-) >> >> Well, !rcu_nocbs is not only used by server but also by pretty much >> everything else, except android IIUC. I can't really measure the whole >> world but I don't see how the idleness of a server/router/desktop/embedded/rt/hpc >> device differs from the idleness of an android device. >> >> But ok I'll try to measure that. > > Although who knows, may be some periodic file operation while idle are specific > to Android. I'll try to trace lazy callbacks while idle and the number of grace > periods associated. One potential usecase could be logging if the logger is opening and closing log files during logging updates. Uladzislau reported this on Android, that a system logger was doing file open/close and triggering RCU that way (file table, dentry, inode code all queue RCU callbacks). Thanks, - Joel
