On Wed, May 11, 2022 at 11:04 PM Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> wrote: > > Hello! > Please find the proof of concept version of call_rcu_lazy() attached. This > gives a lot of savings when the CPUs are relatively idle. Huge thanks to > Rushikesh Kadam from Intel for investigating it with me. > > Some numbers below: > > Following are power savings we see on top of RCU_NOCB_CPU on an Intel platform. > The observation is that due to a 'trickle down' effect of RCU callbacks, the > system is very lightly loaded but constantly running few RCU callbacks very > often. This confuses the power management hardware that the system is active, > when it is in fact idle. > > For example, when ChromeOS screen is off and user is not doing anything on the > system, we can see big power savings. > Before: > Pk%pc10 = 72.13 > PkgWatt = 0.58 > CorWatt = 0.04 > > After: > Pk%pc10 = 81.28 > PkgWatt = 0.41 > CorWatt = 0.03 > > Further, when ChromeOS screen is ON but system is idle or lightly loaded, we > can see that the display pipeline is constantly doing RCU callback queuing due > to open/close of file descriptors associated with graphics buffers. This is > attributed to the file_free_rcu() path which this patch series also touches. > > This patch series adds a simple but effective, and lockless implementation of > RCU callback batching. On memory pressure, timeout or queue growing too big, we > initiate a flush of one or more per-CPU lists. > > Similar results can be achieved by increasing jiffies_till_first_fqs, however > that also has the effect of slowing down RCU. Especially I saw huge slow down > of function graph tracer when increasing that. > > One drawback of this series is, if another frequent RCU callback creeps up in > the future, that's not lazy, then that will again hurt the power. However, I > believe identifying and fixing those is a more reasonable approach than slowing > RCU down for the whole system. > > NOTE: Add debug patch is added in the series toggle /proc/sys/kernel/rcu_lazy > at runtime to turn it on or off globally. It is default to on. Further, please > use the sysctls in lazy.c for further tuning of parameters that effect the > flushing. > > Disclaimer 1: Don't boot your personal system on it yet anticipating power > savings, as TREE07 still causes RCU stalls and I am looking more into that, but > I believe this series should be good for general testing. > > Disclaimer 2: I have intentionally not CC'd other subsystem maintainers (like > net, fs) to keep noise low and will CC them in the future after 1 or 2 rounds > of review and agreements. I did forget to add Disclaimer 3, that this breaks rcu_barrier() and support for that definitely needs work. thanks, - Joel
