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

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Message-ID: <20220901182804.GA108007@lothringen>
Date:   Thu, 1 Sep 2022 20:28:04 +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 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
          <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