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
