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

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Date:   Thu, 1 Sep 2022 09:49:28 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Frederic Weisbecker <frederic@...nel.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 12:07:56PM -0400, Joel Fernandes 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?

							Thanx, Paul