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Message-ID: <20220831152658.GA89704@lothringen>
Date: Wed, 31 Aug 2022 17:26:58 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...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 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
<idle>-0 [007] d..1. 414.468048: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 414.468268: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 414.472268: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 414.476269: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.500577: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.504253: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.508250: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.512249: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.566881: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.568252: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.572249: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 419.576255: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.666873: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.668233: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.672230: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.676232: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.737283: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.740233: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.744230: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 424.748231: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.767922: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.768209: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.772212: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.776212: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.972931: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.976214: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.980211: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [007] d..1. 429.984212: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [003] d..1. 430.402139: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [003] d..1. 430.404211: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [003] d..1. 430.404290: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [003] d..1. 430.408235: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [003] d..1. 430.408304: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
<idle>-0 [003] d..1. 430.412215: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
Thanks.
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