Hello, Joel! Which kernel version have you used for this series? -- Uladzislau Rezki On Thu, May 12, 2022 at 5:18 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > 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 -- Uladzislau Rezki
