Messages in this thread | | | Date | Thu, 13 Jul 2017 21:05:53 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods |
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On Fri, Jul 14, 2017 at 11:47:32AM +0800, Li, Aubrey wrote: > On 2017/7/13 23:20, Paul E. McKenney wrote: > > On Thu, Jul 13, 2017 at 04:53:11PM +0200, Peter Zijlstra wrote: > >> On Thu, Jul 13, 2017 at 10:48:55PM +0800, Li, Aubrey wrote: > >> > >>> - totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs > >>> 9122ns - 15318ns. > >>> ---- In this period(arch idle), rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit > >>> costs 1813ns - 3507ns > >>> > >>> Besides RCU, > >> > >> So Paul wants more details on where RCU hurts so we can try to fix. > > > > More specifically: rcu_needs_cpu(), rcu_prepare_for_idle(), > > rcu_cleanup_after_idle(), rcu_eqs_enter(), rcu_eqs_enter_common(), > > rcu_dynticks_eqs_enter(), do_nocb_deferred_wakeup(), > > rcu_dynticks_task_enter(), rcu_eqs_exit(), rcu_eqs_exit_common(), > > rcu_dynticks_task_exit(), rcu_dynticks_eqs_exit(). > > > > The first three (rcu_needs_cpu(), rcu_prepare_for_idle(), and > > rcu_cleanup_after_idle()) should not be significant unless you have > > CONFIG_RCU_FAST_NO_HZ=y. If you do, it would be interesting to learn > > how often invoke_rcu_core() is invoked from rcu_prepare_for_idle() > > and rcu_cleanup_after_idle(), as this can raise softirq. Also > > rcu_accelerate_cbs() and rcu_try_advance_all_cbs(). > > > > Knowing which of these is causing the most trouble might help me > > reduce the overhead in the current idle path. > > > I don't have details of these functions, I can measure if you want. > Do you have preferred workload for the measurement?
I do not have a specific workload in mind. Could you please choose one with very frequent transitions to and from idle?
> > Also, how big is this system? If you can say, about what is the cost > > of a cache miss to some other CPU's cache? > > > The system has two NUMA nodes. nproc returns 104. local memory access is > ~100 ns and remote memory access is ~200ns, reported by mgen. Does this > address your question?
Very much so, thank you! This will allow me to correctly interpret time spent in the above functions.
Thanx, Paul
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