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
