From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
rushikesh.s.kadam@intel.com, urezki@gmail.com,
neeraj.iitr10@gmail.com, frederic@kernel.org,
rostedt@goodmis.org, vineeth@bitbyteword.org
Subject: Re: [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes
Date: Fri, 8 Jul 2022 04:17:30 +0000 [thread overview]
Message-ID: <Ysev2jbxFGNkLvjG@google.com> (raw)
In-Reply-To: <20220626031206.GJ1790663@paulmck-ThinkPad-P17-Gen-1>
On Sat, Jun 25, 2022 at 08:12:06PM -0700, Paul E. McKenney wrote:
> 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!
Yes this is the package residency amount, not the amount of power. This % is
not power.
> > 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! ;-)
Yes, you are right. Maybe a better way I could put it is it is "lock
contention less" :D
> > 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. ;-)
:-D
thanks,
- Joel
next prev parent reply other threads:[~2022-07-08 4:17 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-22 22:50 [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 1/1] context_tracking: Use arch_atomic_read() in __ct_state for KASAN Joel Fernandes (Google)
2022-06-22 22:58 ` Joel Fernandes
2022-06-22 22:50 ` [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation Joel Fernandes (Google)
2022-06-22 23:18 ` Joel Fernandes
2022-06-26 4:00 ` Paul E. McKenney
2022-06-23 1:38 ` kernel test robot
2022-06-26 4:00 ` Paul E. McKenney
2022-07-08 18:43 ` Joel Fernandes
2022-07-08 23:10 ` Paul E. McKenney
2022-07-10 2:26 ` Joel Fernandes
2022-07-10 16:03 ` Paul E. McKenney
2022-07-12 20:53 ` Joel Fernandes
2022-07-12 21:04 ` Paul E. McKenney
2022-07-12 21:10 ` Joel Fernandes
2022-07-12 22:41 ` Paul E. McKenney
2022-06-29 11:53 ` Frederic Weisbecker
2022-06-29 17:05 ` Paul E. McKenney
2022-06-29 20:29 ` Joel Fernandes
2022-06-29 22:01 ` Frederic Weisbecker
2022-06-30 14:08 ` Joel Fernandes
2022-06-22 22:50 ` [PATCH v2 2/8] rcu: shrinker for lazy rcu Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 3/8] fs: Move call_rcu() to call_rcu_lazy() in some paths Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 4/8] rcu/nocb: Add option to force all call_rcu() to lazy Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 5/8] rcu/nocb: Wake up gp thread when flushing Joel Fernandes (Google)
2022-06-26 4:06 ` Paul E. McKenney
2022-06-26 13:45 ` Joel Fernandes
2022-06-26 13:52 ` Paul E. McKenney
2022-06-26 14:37 ` Joel Fernandes
2022-06-22 22:51 ` [PATCH v2 6/8] rcuscale: Add test for using call_rcu_lazy() to emulate kfree_rcu() Joel Fernandes (Google)
2022-06-23 2:09 ` kernel test robot
2022-06-23 3:00 ` kernel test robot
2022-06-23 8:10 ` kernel test robot
2022-06-26 4:13 ` Paul E. McKenney
2022-07-08 4:25 ` Joel Fernandes
2022-07-08 23:06 ` Paul E. McKenney
2022-07-12 20:27 ` Joel Fernandes
2022-07-12 20:58 ` Paul E. McKenney
2022-07-12 21:15 ` Joel Fernandes
2022-07-12 22:41 ` Paul E. McKenney
2022-06-22 22:51 ` [PATCH v2 7/8] rcu/nocb: Rewrite deferred wake up logic to be more clean Joel Fernandes (Google)
2022-06-22 22:51 ` [PATCH v2 8/8] rcu/kfree: Fix kfree_rcu_shrink_count() return value Joel Fernandes (Google)
2022-06-26 4:17 ` Paul E. McKenney
2022-06-27 18:56 ` Uladzislau Rezki
2022-06-27 20:59 ` Paul E. McKenney
2022-06-27 21:18 ` Joel Fernandes
2022-06-27 21:43 ` Paul E. McKenney
2022-06-28 16:56 ` Joel Fernandes
2022-06-28 21:13 ` Joel Fernandes
2022-06-29 16:56 ` Paul E. McKenney
2022-06-29 19:47 ` Joel Fernandes
2022-06-29 21:07 ` Paul E. McKenney
2022-06-30 14:25 ` Joel Fernandes
2022-06-30 15:29 ` Paul E. McKenney
2022-06-29 16:52 ` Paul E. McKenney
2022-06-26 3:12 ` [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes Paul E. McKenney
2022-07-08 4:17 ` Joel Fernandes [this message]
2022-07-08 22:45 ` Paul E. McKenney
2022-07-10 1:38 ` Joel Fernandes
2022-07-10 15:47 ` Paul E. McKenney
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