On Thu, May 12, 2022 at 8:23 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Wed, May 11, 2022 at 11:17:59PM -0400, Joel Fernandes 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. > > Sometimes OOM conditions result in stalls. I see. > > I did forget to add Disclaimer 3, that this breaks rcu_barrier() and > > support for that definitely needs work. > > Good to know. ;-) > > With this in place, can the system survive a userspace close(open()) > loop, or does that result in OOM? (I am not worried about battery > lifetime while close(open()) is running, just OOM resistance.) Yes, in my testing it survived. I even dropped memory to 512MB and did the open/close loop test. I believe it survives also because we don't let the list to grow too big (other than shrinker flushing). > > Does waiting for the shrinker to kick in suffice, or should the > system pressure be taken into account? As in the "total" numbers > from /proc/pressure/memory. I did not find that taking system memory pressure into account is necessary. > Again, it is very good to see this series! Thanks I appreciate that, I am excited about battery life savings in millions of battery powered devices ;-) Even on my Grand mom's android phone ;-) Thanks, - Joel
