On Thu, Jul 11, 2019 at 12:48:18PM -0400, Joel Fernandes wrote:
> On Thu, Jul 11, 2019 at 08:02:15AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 11, 2019 at 09:08:49AM -0400, Joel Fernandes wrote:
> > > On Thu, Jul 11, 2019 at 05:30:52AM -0700, Paul E. McKenney wrote:
> > > > On Wed, Jul 10, 2019 at 10:20:25AM +0900, Byungchul Park wrote:
> > > > > On Tue, Jul 09, 2019 at 05:41:02AM -0700, Paul E. McKenney wrote:
> > > > > > > Hi Paul,
> > > > > > >
> > > > > > > IMHO, as much as we want to tune the time for fqs to be initiated, we
> > > > > > > can also want to tune the time for the help from scheduler to start.
> > > > > > > I thought only difference between them is a level of urgency. I might be
> > > > > > > wrong. It would be appreciated if you let me know if I miss something.
> > > > > >
> > > > > > Hello, Byungchul,
> > > > > >
> > > > > > I understand that one hypothetically might want to tune this at runtime,
> > > > > > but have you had need to tune this at runtime on a real production
> > > > > > workload? If so, what problem was happening that caused you to want to
> > > > > > do this tuning?
> > > > >
> > > > > Not actually.
> > > > >
> > > > > > > And it's ok even if the patch is turned down based on your criteria. :)
> > > > > >
> > > > > > If there is a real need, something needs to be provided to meet that
> > > > > > need. But in the absence of a real need, past experience has shown
> > > > > > that speculative tuning knobs usually do more harm than good. ;-)
> > > > >
> > > > > It makes sense, "A speculative tuning knobs do more harm than good".
> > > > >
> > > > > Then, it would be better to leave jiffies_till_{first,next}_fqs tunnable
> > > > > but jiffies_till_sched_qs until we need it.
> > > > >
> > > > > However,
> > > > >
> > > > > (1) In case that jiffies_till_sched_qs is tunnable:
> > > > >
> > > > > We might need all of jiffies_till_{first,next}_qs,
> > > > > jiffies_till_sched_qs and jiffies_to_sched_qs because
> > > > > jiffies_to_sched_qs can be affected by any of them. So we
> > > > > should be able to read each value at any time.
> > > > >
> > > > > (2) In case that jiffies_till_sched_qs is not tunnable:
> > > > >
> > > > > I think we don't have to keep the jiffies_till_sched_qs any
> > > > > longer since that's only for setting jiffies_to_sched_qs at
> > > > > *booting time*, which can be done with jiffies_to_sched_qs too.
> > > > > It's meaningless to keep all of tree variables.
> > > > >
> > > > > The simpler and less knobs that we really need we have, the better.
> > > > >
> > > > > what do you think about it?
> > > > >
> > > > > In the following patch, I (1) removed jiffies_till_sched_qs and then
> > > > > (2) renamed jiffies_*to*_sched_qs to jiffies_*till*_sched_qs because I
> > > > > think jiffies_till_sched_qs is a much better name for the purpose. I
> > > > > will resend it with a commit msg after knowing your opinion on it.
> > > >
> > > > I will give you a definite "maybe".
> > > >
> > > > Here are the two reasons for changing RCU's embarrassingly large array
> > > > of tuning parameters:
> > > >
> > > > 1. They are causing a problem in production. This would represent a
> > > > bug that clearly must be fixed. As you say, this change is not
> > > > in this category.
> > > >
> > > > 2. The change simplifies either RCU's code or the process of tuning
> > > > RCU, but without degrading RCU's ability to run everywhere and
> > > > without removing debugging tools.
> > > >
> > > > The change below clearly simplifies things by removing a few lines of
> > > > code, and it does not change RCU's default self-configuration. But are
> > > > we sure about the debugging aspect? (Please keep in mind that many more
> > > > sites are willing to change boot parameters than are willing to patch
> > > > their kernels.)
> > > >
> > > > What do you think?
> > >
> > > Just to add that independent of whether the runtime tunable make sense or
> > > not, may be it is still worth correcting the 0444 to be 0644 to be a separate
> > > patch?
> >
> > You lost me on this one. Doesn't changing from 0444 to 0644 make it be
> > a runtime tunable?
>
> I was going by our earlier discussion that the parameter is still writable at
> boot time. You mentioned something like the following:
> ---
> In Byungchul's defense, the current module_param() permissions are
> 0444, which really is read-only. Although I do agree that they can
> be written at boot, one could use this same line of reasoning to argue
> that const variables can be written at compile time (or, for on-stack
> const variables, at function-invocation time). But we still call them
> "const".
> ---
>
> Sorry if I got confused. You are right that we could leave it as read-only.
>
> > > > Finally, I urge you to join with Joel Fernandes and go through these
> > > > grace-period-duration tuning parameters. Once you guys get your heads
> > > > completely around all of them and how they interact across the different
> > > > possible RCU configurations, I bet that the two of you will have excellent
> > > > ideas for improvement.
> > >
> > > Yes, I am quite happy to join forces. Byungchul, let me know what about this
> > > or other things you had in mind. I have some other RCU topics too I am trying
> > > to get my head around and planning to work on more patches.
> > >
> > > Paul, in case you had any other specific tunables or experiments in mind, let
> > > me know. I am quite happy to try out new experiments and learn something
> > > based on tuning something.
> >
> > These would be the tunables controlling how quickly RCU takes its
> > various actions to encourage the current grace period to end quickly.
> > I would be happy to give you the exact list if you wish, but most of
> > them have appeared in this thread.
> >
> > The experiments should be designed to work out whether the current
> > default settings have configurations where they act badly. This might
> > also come up with advice for people attempting hand-tuning, or proposed
> > parameter-checking code to avoid bad combinations.
> >
> > For one example, setting the RCU CPU stall timeout too low will definitely
> > cause some unwanted splats. (Yes, one could argue that other things in
> > the kernel should change to allow this value to decrease, but things
> > like latency tracer and friends are probably more useful and important.)
>
> Ok, thank you for the hints.
Hmm, speaking of grace period durations, it seems to me the maximum grace
period ever is recorded in rcu_state.gp_max. However it is not read from
anywhere.
Any idea why it was added but not used?
I am interested in dumping this value just for fun, and seeing what I get.
I wonder also it is useful to dump it in rcutorture/rcuperf to find any
issues, or even expose it in sys/proc fs to see what worst case grace periods
look like.
- Joel
