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Date:   Wed, 31 Aug 2022 17:26:58 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@intel.com>,
        "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
        Neeraj upadhyay <neeraj.iitr10@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        rcu <rcu@vger.kernel.org>,
        Vineeth Pillai <vineeth@bitbyteword.org>
Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous
 fixes
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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.