On Wed, Jun 22, 2022 at 10:50:53PM +0000, Joel Fernandes (Google) wrote: > > Hello! > Please find the next improved version of call_rcu_lazy() attached. The main > difference between the previous version is that it is now using bypass lists, > and thus handling rcu_barrier() and hotplug situations, with some small changes > to those parts. > > I also don't see the TREE07 RCU stall from v1 anymore. > > In the v1, we some numbers below (testing on v2 is in progress). Rushikesh, > feel free to pull these patches into your tree. Just to note, you will also > need to pull the call_rcu_lazy() user patches from v1. I have dropped in this > series, just to make the series focus on the feature code first. > > 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 So not quite 30% savings in power at the package level? Not bad at all! > 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. It is no longer lockless, correct? Or am I missing something subtle? Full disclosure: I don't see a whole lot of benefit to its being lockless. But truth in advertising! ;-) > 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. Very good! I have you down as the official call_rcu_lazy() whack-a-mole developer. ;-) Thanx, Paul > Disclaimer: 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) (7): > rcu: Introduce call_rcu_lazy() API implementation > fs: Move call_rcu() to call_rcu_lazy() in some paths > rcu/nocb: Add option to force all call_rcu() to lazy > rcu/nocb: Wake up gp thread when flushing > rcuscale: Add test for using call_rcu_lazy() to emulate kfree_rcu() > rcu/nocb: Rewrite deferred wake up logic to be more clean > rcu/kfree: Fix kfree_rcu_shrink_count() return value > > Vineeth Pillai (1): > rcu: shrinker for lazy rcu > > fs/dcache.c | 4 +- > fs/eventpoll.c | 2 +- > fs/file_table.c | 2 +- > fs/inode.c | 2 +- > include/linux/rcu_segcblist.h | 1 + > include/linux/rcupdate.h | 6 + > kernel/rcu/Kconfig | 8 ++ > kernel/rcu/rcu.h | 8 ++ > kernel/rcu/rcu_segcblist.c | 19 +++ > kernel/rcu/rcu_segcblist.h | 24 ++++ > kernel/rcu/rcuscale.c | 64 +++++++++- > kernel/rcu/tree.c | 35 +++++- > kernel/rcu/tree.h | 10 +- > kernel/rcu/tree_nocb.h | 217 +++++++++++++++++++++++++++------- > 14 files changed, 345 insertions(+), 57 deletions(-) > > -- > 2.37.0.rc0.104.g0611611a94-goog >
