Message-ID: <20190712130105.GL26519@linux.ibm.com>
Date:   Fri, 12 Jul 2019 06:01:05 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Byungchul Park <byungchul.park@....com>, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

On Thu, Jul 11, 2019 at 03:58:39PM -0400, Joel Fernandes wrote:
> 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?

If I remember correclty, it used to be used in debugfs prints.  It is
useful for working out how low you can decrease rcutorture.stall_cpu to
without getting RCU CPU stall warnings.  A rather infrequent need,
given that the mainline default has been adjusted only once.

> 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.

That might be worthwhile.

							Thanx, Paul

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