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

Li, Aubrey
2017-07-14T03:47:37Z

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