Re: [RFC] rcu: Check the range of jiffies_till_xxx_fqs on setting them

[Byungchul Park]

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.

> 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. :(

> But I can see the desire to share code.
>
> One approach would be to embed the kernel_params_ops structure inside
> another structure containing the limits, then just have two structures.
> Perhaps something like this already exists?  I don't see it right off,
> but then again, I am not exactly an expert on module_param.

It would be much nicer if we can as you said. I will check it.

Thanks a lot Paul.

--
Thanks,
Byungchul