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! Thanks.
