linux-kernel - Re: [PATCH v2 rcu/dev 1/2] rcu: Track laziness during boot and suspend

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Message-ID: <CAEXW_YSNurO-hK+q2amP6wa96jr0KkZ_ggF+5x_sTHESC9vpNw@mail.gmail.com>
Date:   Sun, 15 Jan 2023 16:34:58 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        fweisbec@...il.com, urezki@...il.com
Subject: Re: [PATCH v2 rcu/dev 1/2] rcu: Track laziness during boot and suspend

On Sun, Jan 15, 2023 at 4:25 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 12 Jan 2023 00:52:22 +0000
> "Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:
>
> > -- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -144,8 +144,45 @@ bool rcu_gp_is_normal(void)
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
> >
> > -static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
> > +static atomic_t rcu_async_hurry_nesting = ATOMIC_INIT(1);
> > +/*
> > + * Should call_rcu() callbacks be processed with urgency or are
> > + * they OK being executed with arbitrary delays?
> > + */
> > +bool rcu_async_should_hurry(void)
> > +{
> > +     return !IS_ENABLED(CONFIG_RCU_LAZY) ||
> > +            atomic_read(&rcu_async_hurry_nesting);
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_async_should_hurry);
> > +
> > +/**
> > + * rcu_async_hurry - Make future async RCU callbacks not lazy.
> > + *
> > + * After a call to this function, future calls to call_rcu()
> > + * will be processed in a timely fashion.
> > + */
> > +void rcu_async_hurry(void)
> > +{
> > +     if (IS_ENABLED(CONFIG_RCU_LAZY))
> > +             atomic_inc(&rcu_async_hurry_nesting);
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_async_hurry);
> >
>
> Where do you plan on calling these externally, as they are being
> marked exported?
>
> If you allow random drivers to enable this, I can see something
> enabling it and hitting an error path that causes it to never disable
> it.

You mean, just like rcu_expedite_gp() ?

> I wouldn't have EXPORT_SYMBOL_GPL() unless you really know that it is
> needed externally.

At the moment it is not called externally but in the future, it could
be from rcutorture. If you see rcu_expedite_gp(), that is exported
too. I was just modeling it around that API.

thanks,

 - Joel

>
> -- Steve
>
>
> > +/**
> > + * rcu_async_relax - Make future async RCU callbacks lazy.
> > + *
> > + * After a call to this function, future calls to call_rcu()
> > + * will be processed in a lazy fashion.
> > + */
> > +void rcu_async_relax(void)
> > +{
> > +     if (IS_ENABLED(CONFIG_RCU_LAZY))
> > +             atomic_dec(&rcu_async_hurry_nesting);
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_async_relax);
> > +
> > +static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);