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Date: Sun, 10 Mar 2024 11:56:02 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH 15/30] rcu: handle quiescent states for PREEMPT_RCU=n,
 PREEMPT_COUNT=y
Message-ID: <da51dacc-cdf7-4129-b424-b32764736f48@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20240213055554.1802415-1-ankur.a.arora@oracle.com>
 <20240213055554.1802415-16-ankur.a.arora@oracle.com>
 <20240310100330.GA2705505@joelbox2>
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In-Reply-To: <20240310100330.GA2705505@joelbox2>

On Sun, Mar 10, 2024 at 06:03:30AM -0400, Joel Fernandes wrote:
> Hello Ankur and Paul,
> 
> On Mon, Feb 12, 2024 at 09:55:39PM -0800, Ankur Arora wrote:
> > With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
> > states for read-side critical sections via rcu_all_qs().
> > One reason why this was necessary: lacking preempt-count, the tick
> > handler has no way of knowing whether it is executing in a read-side
> > critical section or not.
> > 
> > With PREEMPT_AUTO=y, there can be configurations with (PREEMPT_COUNT=y,
> > PREEMPT_RCU=n). This means that cond_resched() is a stub which does
> > not provide for quiescent states via rcu_all_qs().
> > 
> > So, use the availability of preempt_count() to report quiescent states
> > in rcu_flavor_sched_clock_irq().
> > 
> > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> > ---
> >  kernel/rcu/tree_plugin.h | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 26c79246873a..9b72e9d2b6fe 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -963,13 +963,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
> >   */
> >  static void rcu_flavor_sched_clock_irq(int user)
> >  {
> > -	if (user || rcu_is_cpu_rrupt_from_idle()) {
> > +	if (user || rcu_is_cpu_rrupt_from_idle() ||
> > +	    (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
> > +	     !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)))) {
> 
> I was wondering if it makes sense to even support !PREEMPT_RCU under
> CONFIG_PREEMPT_AUTO.
> 
> AFAIU, this CONFIG_PREEMPT_AUTO series preempts the kernel on
> the next tick boundary in the worst case, with all preempt modes including
> the preempt=none mode.
> 
> Considering this, does it makes sense for RCU to be non-preemptible in
> CONFIG_PREEMPT_AUTO=y? Because if that were the case, and a read-side critical
> section extended beyond the tick, then it prevents the PREEMPT_AUTO preemption
> from happening, because rcu_read_lock() would preempt_disable().

Yes, it does make sense for RCU to be non-preemptible in kernels
built with CONFIG_PREEMPT_AUTO=y and either CONFIG_PREEMPT_NONE=y or
CONFIG_PREEMPT_VOLUNTARY=y.  As noted in earlier discussions, there are
systems that are adequately but not abundantly endowed with memory.
Such systems need non-preemptible RCU to avoid preempted-reader OOMs.
Note well that non-preemptible RCU explicitly disables preemption across
all RCU readers.

							Thanx, Paul


> To that end, I wonder if CONFIG_PREEMPT_AUTO should select CONFIG_PREEMPTION
> (or CONFIG_PREEMPT_BUILD, not sure which) as well because it does cause
> kernel preemption. That then forces selection of CONFIG_PREEMPT_RCU as well.
> 
> thanks,
> 
>  - Joel
> 
> 
> 
> 
> 
> 
> 
> >  
> >  		/*
> >  		 * Get here if this CPU took its interrupt from user
> > -		 * mode or from the idle loop, and if this is not a
> > -		 * nested interrupt.  In this case, the CPU is in
> > -		 * a quiescent state, so note it.
> > +		 * mode, from the idle loop without this being a nested
> > +		 * interrupt, or while not holding a preempt count.
> > +		 * In this case, the CPU is in a quiescent state, so note
> > +		 * it.
> >  		 *
> >  		 * No memory barrier is required here because rcu_qs()
> >  		 * references only CPU-local variables that other CPUs
> > -- 
> > 2.31.1
> >