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

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Date:   Thu, 1 Sep 2022 16:58:19 +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 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!

Thanks.