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?
> 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? Thanks, -Aubrey