On Mon, Jun 13, 2022 at 06:53:27PM +0000, Joel Fernandes wrote: > 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! Looking forward to seeing it! Thanx, Paul > 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 > >
