On Thu, May 12, 2022 at 03:04:28AM +0000, Joel Fernandes (Google) 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. Just a status update, we're reworking this code to use the bypass lists which takes care of a lot of corner cases. I've had to make changes the the rcuog thread code to handle wakes up a bit differently and such. Initial testing is looking good. Currently working on the early-flushing of the lazy CBs based on memory pressure. Should be hopefully sending v2 soon! thanks, - Joel > > 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. > > Joel Fernandes (Google) (14): > rcu: Add a lock-less lazy RCU implementation > workqueue: Add a lazy version of queue_rcu_work() > block/blk-ioc: Move call_rcu() to call_rcu_lazy() > cred: Move call_rcu() to call_rcu_lazy() > fs: Move call_rcu() to call_rcu_lazy() in some paths > kernel: Move various core kernel usages to call_rcu_lazy() > security: Move call_rcu() to call_rcu_lazy() > net/core: Move call_rcu() to call_rcu_lazy() > lib: Move call_rcu() to call_rcu_lazy() > kfree/rcu: Queue RCU work via queue_rcu_work_lazy() > i915: Move call_rcu() to call_rcu_lazy() > rcu/kfree: remove useless monitor_todo flag > rcu/kfree: Fix kfree_rcu_shrink_count() return value > DEBUG: Toggle rcu_lazy and tune at runtime > > block/blk-ioc.c | 2 +- > drivers/gpu/drm/i915/gem/i915_gem_object.c | 2 +- > fs/dcache.c | 4 +- > fs/eventpoll.c | 2 +- > fs/file_table.c | 3 +- > fs/inode.c | 2 +- > include/linux/rcupdate.h | 6 + > include/linux/sched/sysctl.h | 4 + > include/linux/workqueue.h | 1 + > kernel/cred.c | 2 +- > kernel/exit.c | 2 +- > kernel/pid.c | 2 +- > kernel/rcu/Kconfig | 8 ++ > kernel/rcu/Makefile | 1 + > kernel/rcu/lazy.c | 153 +++++++++++++++++++++ > kernel/rcu/rcu.h | 5 + > kernel/rcu/tree.c | 28 ++-- > kernel/sysctl.c | 23 ++++ > kernel/time/posix-timers.c | 2 +- > kernel/workqueue.c | 25 ++++ > lib/radix-tree.c | 2 +- > lib/xarray.c | 2 +- > net/core/dst.c | 2 +- > security/security.c | 2 +- > security/selinux/avc.c | 4 +- > 25 files changed, 255 insertions(+), 34 deletions(-) > create mode 100644 kernel/rcu/lazy.c > > -- > 2.36.0.550.gb090851708-goog >
