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Date:   Tue, 12 Dec 2023 17:32:57 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Andrea Righi <andrea.righi@...onical.com>,
        John Stultz <jstultz@...gle.com>, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Provide a boot time parameter to control lazy RCU

On Tue, Dec 12, 2023 at 7:05 AM Qais Yousef <qyousef@...alina.io> wrote:
>
> On 12/09/23 01:26, Joel Fernandes wrote:
> > On 12/7/23 12:20, Qais Yousef wrote:
> > > On 12/05/23 16:20, Joel Fernandes wrote:
> > >
> > >> I think a better approach is not do an anti-CONFIG option and instead do
> > >> a shorter parameter "rcutree.lazy=0". If CONFIG_RCU_LAZY is set, then we can
> > >> just default to keeping lazy on. I'd like to avoid proliferation of already
> > >> large number of RCU config options and more chances of errors.
> > >
> > > The issue is that we don't want to ship with default on :-)
> >
> > Yes, so you can ship with rcutree.enable_lazy=0 which this patch adds, no? In
> > theory, you can accomplish this by simply CONFIG_RCU_LAZY=y and
> > rcutree.enable_lazy=0 or rcutree.lazy=0.
> >
> > However, I see the inconvenience factor (you have to set a boot parameter
> > without making this a purely .config affair) so I am not terribly opposed with
> > this patch (I am also guilty of adding a CONFIG option to avoid having to set a
> > boot parameter (for unrelated feature), but in my defense I did not know a boot
> > parameter existed for the said feature). ;-)
>
> It is more than inconvenience. The GKI doesn't ship with a specific userspace.
> So we can't guarantee the boot parameter will be set and have to rely on no one
> missing the memo to add this additional parameter.

Yes, I see that now. Looks like Android also needs to be supplying a
"GKI boot parameter" requirement to their GKI supplied kernels ;-).
But I see the issue you are referring to now.

It would be good to add these details to your respin.

> > > Not allowing this
> > > in upstream means I'll either have to resort to keep it disabled, or carry out
> > > of tree patch to get what I want. Both of which would be unfortunate.
> >
> > There is already precedent for building things into the kernel but keeping them
> > default off, so I don't have an issue with the experimentation use case. I was
> > just discussing whether the additional CONFIG is really needed when you already
> > have added a boot param to keep it default-off. If you have an argument for why
> > that would be really helpful [1].
> >
> > Also, nit: rcutree.enable_lazy is probably better than rcutree.enable_rcu_lazy.
> > The 'rcu' is redundant.
>
> It matches the config option so feels natural to have them both named the same?

Ok, either is fine with me.

> > Other than that, the patch LGTM but if you could update the commit log with
> > details about [1], that would be great. And while at it, you could add my tag:
>
> You forgot to include [1]? Or I'm just blind today?

Heh, if you search "[1]" you see it above where I said "helpful". ;-).
But apologies if I caused confusion.

thanks,

 - Joel


>
> >
> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>
> Thanks!
>
> --
> Qais Yousef

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