linux-kernel - Re: [PATCH v4 00/14] Implement call_rcu

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Date:   Thu, 1 Sep 2022 07:39:07 -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 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?

							Thanx, Paul

>           <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.