linux-kernel - Re: [PATCH RFC tip/core/rcu 1/2] rcu: Defer reporting RCU-preempt quiescent states when disabled

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Message-ID: <20180702003732.GB95395@joelaf.mtv.corp.google.com>
Date:   Sun, 1 Jul 2018 17:37:32 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com
Subject: Re: [PATCH RFC tip/core/rcu 1/2] rcu: Defer reporting RCU-preempt
 quiescent states when disabled

On Sun, Jul 01, 2018 at 03:25:01PM -0700, Paul E. McKenney wrote:
[...]
> > > @@ -602,6 +589,66 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > >  	}
> > >  }
> > >  
> > > +/*
> > > + * Is a deferred quiescent-state pending, and are we also not in
> > > + * an RCU read-side critical section?  It is the caller's responsibility
> > > + * to ensure it is otherwise safe to report any deferred quiescent
> > > + * states.  The reason for this is that it is safe to report a
> > > + * quiescent state during context switch even though preemption
> > > + * is disabled.  This function cannot be expected to understand these
> > > + * nuances, so the caller must handle them.
> > > + */
> > > +static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > +{
> > > +	return (this_cpu_ptr(&rcu_preempt_data)->deferred_qs ||
> > > +		READ_ONCE(t->rcu_read_unlock_special.s)) &&
> > > +	       !t->rcu_read_lock_nesting;
> > > +}
> > > +
> > > +/*
> > > + * Report a deferred quiescent state if needed and safe to do so.
> > > + * As with rcu_preempt_need_deferred_qs(), "safe" involves only
> > > + * not being in an RCU read-side critical section.  The caller must
> > > + * evaluate safety in terms of interrupt, softirq, and preemption
> > > + * disabling.
> > > + */
> > > +static void rcu_preempt_deferred_qs(struct task_struct *t)
> > > +{
> > > +	unsigned long flags;
> > > +
> > > +	if (!rcu_preempt_need_deferred_qs(t))
> > > +		return;
> > > +	local_irq_save(flags);
> > > +	rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > +}
> > > +
> > > +/*
> > > + * Handle special cases during rcu_read_unlock(), such as needing to
> > > + * notify RCU core processing or task having blocked during the RCU
> > > + * read-side critical section.
> > > + */
> > > +static void rcu_read_unlock_special(struct task_struct *t)
> > > +{
> > > +	unsigned long flags;
> > > +	bool preempt_bh_were_disabled = !!(preempt_count() & ~HARDIRQ_MASK);
> > 
> > Would it be better to just test for those bits just to be safe the higher
> > order bits don't bleed in, such as PREEMPT_NEED_RESCHED, something like the
> > following based on the 'dev' branch?
> 
> Good point!  My plan is to merge it into the original commit with
> attribution.  Please let me know if you have objections.
> 

Sure! That sounds good to me.

thanks!

- Joel