Date:   Mon, 9 May 2022 13:17:00 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Alison Chaiken <achaiken@...ora.tech>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraj.iitr10@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH] rcu/nocb: Add an option to ON/OFF an offloading from RT context

On Sun, May 8, 2022 at 11:37 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Sun, May 08, 2022 at 08:17:49PM -0400, Joel Fernandes wrote:
> > On Sun, May 8, 2022 at 5:32 PM Paul E. McKenney <paulmck@...nel.org> wrote:
 [...]
> > > > Also, I think it is wrong to assume that a certain kind of system will
> > > > always have a certain number of callbacks to process at a time. That
> > > > seems prone to poor design due to assumptions which may not always be
> > > > true.
> > >
> > > Who was assuming that?  Uladzislau was measuring rather than assuming,
> > > if that was what you were getting at.  Or if you are thinking about
> > > things like qhimark, your point is exactly why there is both a default
> > > (which has worked quite well for a very long time) and the ability to
> > > adjust based on the needs of your specific system.
> >
> > I was merely saying that based on measurements make assumptions, but
> > in the real world the assumption may not be true, then everything
> > falls apart. Instead I feel, callback threads should be RT only if 1.
> > As you mentioned, the time based thing. 2. If the CB list is long and
> > there's lot of processing. But instead, if it is made a CONFIG option,
> > then that forces a fixed behavior which may fall apart in the real
> > world. I think adding more CONFIGs and special cases is more complex
> > but that's my opinion.
>
> Again, exactly what problem are you trying to solve?
>
> From what I can see, Uladzislau's issue can be addressed by statically
> setting the rcuo kthreads to SCHED_OTHER at boot time.  The discussion
> is on exactly how RCU is to be informed of this, at kernel build time.
>
> > > > Can we not have 2 sets of RCU offload threads, one which operate at RT
> > > > and only process few callbacks at a time, while another which is the
> > > > lower priority CFS offload thread - executes whenever there is a lot
> > > > of CBs pending? Just a thought.
> > >
> > > How about if we start by solving the problems we know that we have?
> >
> > I don't know why you would say that, because we are talking about
> > solving the specific problem Vlad's patch addresses, not random
> > problems. Which is that, Android wants to run expedited GPs, but when
> > the callback list is large, the RT nocb thread can starve other
> > things. Did I misunderstand the patch? If so, sorry about that but
> > that's what my email was discussing. i.e. running of CBs in RT
> > threads. I suck at writing well as I clearly miscommunicated.
>
> OK.
>
> Why do you believe that this needs anything other than small adjustments
> the defaults of existing Kconfig options?  Or am I completely missing
> the point of your proposal?
>
> > > > Otherwise, I feel like we might be again proliferating CONFIG options
> > > > and increasing burden on the user to get it the CONFIG right.
> > >
> > > I bet that we will solve this without adding any new Kconfig options.
> > > And I bet that the burden is at worst on the device designer, not on
> > > the user.  Plus it is entirely possible that there might be a way to
> > > automatically configure things to handle what we know about today,
> > > again without adding Kconfig options.
> >
> > Yes, agreed.
>
> If I change my last sentence to read as follows, are we still in
> agreement?
>
>         Plus it is entirely possible that there might be a way to
>         automatically configure things to handle what we know about today,
>         again without adding Kconfig options and without changing runtime
>         code beyond that covered by Uladzislau's patch.

Yes, actually the automatic configuration of things is what I meant,
that's the "problem" I was referring to, where the system does the
right thing for a broader range of systems, without requiring the
users to find RCU issues and hand-tune them (that requires said users
to have tracing and debugging skills and get lucky finding a problem).
To be fair, I did not propose any solutions to such problems either,
it is just some ideas. I don't like knobs too much and I don't trust
users or system designers to get them right most of the time.

In that sense,  I don't think making rcuo threads run as RT or not
(which this patch does) is really fixing the problems. In one case,
you might have priority inversion, in another case you might cause
starvation. Probably what is needed is best of both worlds. That said,
I don't have better solutions right now than what I mentioned, which
is to assign priorities to the callbacks themselves and run them in
threads of different priorities.

For the record, I am not against the patch or anything like that (and
even if I was, I am not sure that it matters for merging :P)

Thanks,

 - Joel

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