Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes

[Paul E. McKenney]

On Thu, Sep 01, 2022 at 08:28:04PM +0200, Frederic Weisbecker wrote:
> 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

But reducing ticks is not the only way energy is saved.  The other way
is a reduction in the number of grace periods.  One way to estimate this
is to take the per-second grace period rate and subtract one grace period
per two seconds.  If the system is idle, this effect might be significant.

							Thanx, Paul

>           <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