linux-kernel - Re: [PATCH] rcu: Make jiffies_till_sched

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Message-ID: <20190709064531.GA15734@X58A-UD3R>
Date:   Tue, 9 Jul 2019 15:45:31 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.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 Tue, Jul 09, 2019 at 02:58:16PM +0900, Byungchul Park wrote:
> On Mon, Jul 08, 2019 at 09:03:59AM -0400, Joel Fernandes wrote:
> > > Actually, the intent was to only allow this to be changed at boot time.
> > > Of course, if there is now a good reason to adjust it, it needs
> > > to be adjustable.  So what situation is making you want to change
> > > jiffies_till_sched_qs at runtime?  To what values is it proving useful
> > > to adjust it?  What (if any) relationships between this timeout and the
> > > various other RCU timeouts need to be maintained?  What changes to
> > > rcutorture should be applied in order to test the ability to change
> > > this at runtime?
> > 
> > I am also interested in the context, are you changing it at runtime for
> > experimentation? I recently was doing some performance experiments and it is
> > quite interesting how reducing this value can shorten grace period times :)
> 
> Hi Joel,
> 
> I've read a thread talking about your experiment to see how the grace
> periods change depending on the tunnable variables which was interesting
> to me. While reading it, I found out jiffies_till_sched_qs is not
> tunnable at runtime unlike jiffies_till_{first,next}_fqs which looks
> like non-sense to me that's why I tried this patch. :)
> 
> 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.

Let me mention one more thing here...

This is about jiffies_till_sched_qs, I think, used to directly set
jiffies_to_sched_qs.

> I thought only difference between them is a level of urgency.

Of course, they are coupled in case jiffies_to_sched_qs is calculated
from jiffies_till_{first,next}_fqs though, I'm just talking about its
original motivation or concept.

Thanks,
Byungchul