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! Interesting! - Joel
