Re: [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes

[Paul E. McKenney]

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
>