linux-kernel - Re: [PATCH v2 0/8] Implement call_rcu

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Date:   Fri, 8 Jul 2022 04:17:30 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, rostedt@...dmis.org,
        vineeth@...byteword.org
Subject: Re: [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes

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