Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

Paul E. McKenney
2017-07-14T04:06:03Z

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