linux-kernel - Re: [RFC] rcu: Check the range of jiffies_till_xxx

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Date:   Thu, 31 May 2018 10:27:46 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     jiangshanlai@...il.com, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com, joel@...lfernandes.org
Subject: Re: [RFC] rcu: Check the range of jiffies_till_xxx_fqs on setting
 them



On 2018-05-30 22:49, Paul E. McKenney wrote:
> On Wed, May 30, 2018 at 10:06:52PM +0900, Byungchul Park wrote:
>>
>>
>> On 2018-05-29 21:01, Paul E. McKenney wrote:
>>> On Tue, May 29, 2018 at 04:23:36PM +0900, Byungchul Park wrote:
>>>> Hello Paul and folks,
>>>>
>>>> I've thought the code should've been like the below since the range
>>>> checking of jiffies_till_first_fqs and jiffies_till_next_fqs everytime
>>>> in the loop of rcu_gp_kthread are unnecessary at all. However, it's ok
>>>> even if you don't think it's worth doing it.
>>>
>>> Nice!
>>>
>>>> Secondly, I also think jiffies_till_first_fqs = 0 is meaningless so
>>>> added checking and adjusting it as what's done on jiffies_till_next_fqs.
>>>> Thought?
>>>
>>> Actually, jiffies_till_first_fqs == 0 is very useful for cases where
>>> at least one CPU is expected to be idle and grace-period latency is
>>> important.  In this case, doing the first scan immediately gets the
>>> dyntick-idle state recorded immediately, getting the idle CPUs out of
>>> the way of the grace period immediately.
>>
>> Hi Paul~
>>
>> You might want to handle it through sysfs. Otherwise, we can do it with
>> force_quiescent_state() IMHO.
> 
> I agree that sysfs would be better than debugfs because these parameters
> are about tuning, not debugging, so good point!
> 
>>> So why not do this scan as part of grace-period initialization?  Because
>>> doing so consumes extra CPU and results in extra cache misses, which is
>>> the opposite of what you want on a completely busy system, especially
>>> one where the CPUs are context switching quickly.  Thus no scan during
>>> grace-period initialization.
>>
>> I am sorry I don't understand this paragraph. :(
> 
> Let me try again.  ;-)
> 
> I could change RCU to avoid the need for jiffies_till_first_fqs == 0,
> but doing that would increase CPU consumption for workloads that are
> already bottlenecked on the CPU.  So I won't be making that change,
> so we still need jiffies_till_first_fqs == 0.

Thanks. I see. We need it then.

-- 
Thanks,
Byungchul