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Message-ID: <20220901203656.GD6159@paulmck-ThinkPad-P17-Gen-1>
Date: Thu, 1 Sep 2022 13:36:56 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
LKML <linux-kernel@...r.kernel.org>,
Rushikesh S Kadam <rushikesh.s.kadam@...el.com>,
"Uladzislau Rezki (Sony)" <urezki@...il.com>,
Neeraj upadhyay <neeraj.iitr10@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
rcu <rcu@...r.kernel.org>,
Vineeth Pillai <vineeth@...byteword.org>
Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous
fixes
On Thu, Sep 01, 2022 at 08:28:04PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 01, 2022 at 09:49:28AM -0700, Paul E. McKenney wrote:
> > > On 9/1/2022 10:58 AM, Frederic Weisbecker wrote:
> > > > On Thu, Sep 01, 2022 at 07:39:07AM -0700, Paul E. McKenney wrote:
> > > >> On Wed, Aug 31, 2022 at 05:26:58PM +0200, Frederic Weisbecker wrote:
> > > >>> On Tue, Aug 30, 2022 at 09:46:34AM -0700, Paul E. McKenney wrote:
> > > >>>>> Although who knows, may be some periodic file operation while idle are specific
> > > >>>>> to Android. I'll try to trace lazy callbacks while idle and the number of grace
> > > >>>>> periods associated.
> > > >>>>
> > > >>>> Sounds like a good start.
> > > >>>>
> > > >>>> And yes, we don't need to show that the whole !NOCB world needs this,
> > > >>>> just some significant portion of it. But we do need some decent evidence.
> > > >>>> After all, it is all too easy to do a whole lot of work and find that
> > > >>>> the expected benefits fail to materialize.
> > > >>>
> > > >>> So here is some quick test. I made a patch that replaces Joel's 1st patch
> > > >>> with an implementation of call_rcu_lazy() that queues lazy callbacks
> > > >>> through the regular call_rcu() way but it counts them in a lazy_count.
> > > >>>
> > > >>> Upon idle entry it reports whether the tick is retained solely by lazy
> > > >>> callbacks or not.
> > > >>>
> > > >>> I get periodic and frequent results on my idle test box, something must be
> > > >>> opening/closing some file periodically perhaps.
> > > >>>
> > > >>> Anyway the thing can be tested with this branch:
> > > >>>
> > > >>> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > > >>> rcu/lazy-trace
> > > >>>
> > > >>> Excerpt:
> > > >>>
> > > >>> <idle>-0 [007] d..1. 414.226966: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> > > >>> <idle>-0 [007] d..1. 414.228271: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> > > >>> <idle>-0 [007] d..1. 414.232269: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> > > >>> <idle>-0 [007] d..1. 414.236269: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> > > >>
> > > >> Just to make sure that I understand, at this point, there is only the
> > > >> one lazy callback (and no non-lazy callbacks) on this CPU, and that
> > > >> CPU is therefore keeping the tick on only for the benefit of that one
> > > >> lazy callback. And for the above four traces, this is likely the same
> > > >> lazy callback.
> > > >>
> > > >> Did I get it right, or is there something else going on?
> > > >
> > > > Exactly that!
> >
> > Are these callbacks confined to the RCU_NEXT_READY_TAIL and RCU_NEXT_TAIL
> > segments, which are the ones that could (in theory) buffer callbacks
> > without having started a grace period? Or is it all the callbacks
> > regardless of segment?
>
> Ah good point!
>
> So I just excluded when those segments have callbacks and I now only get
> two tick retains every two seconds:
>
> <idle>-0 [007] d..1. 1111.893649: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1111.967575: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
But reducing ticks is not the only way energy is saved. The other way
is a reduction in the number of grace periods. One way to estimate this
is to take the per-second grace period rate and subtract one grace period
per two seconds. If the system is idle, this effect might be significant.
Thanx, Paul
> <idle>-0 [007] d..1. 1113.895470: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1115.669446: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1115.898144: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1117.202833: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1117.900521: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1119.903327: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1120.766864: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1121.909182: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1122.441927: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1123.908911: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1125.868505: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1125.910898: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1127.682837: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1127.913719: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1129.916740: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1130.967052: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1131.919256: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1132.957163: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [000] d..1. 1133.630082: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1133.923053: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1135.927054: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1136.067679: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1137.652294: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1137.932546: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1138.200768: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1139.932573: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1141.167489: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1141.935232: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1143.440538: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
> <idle>-0 [007] d..1. 1143.938560: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
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