Re: [PATCH rcu 03/10] rcu: Add QS check in rcu_exp_handler() for non-preemptible kernels

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Aug 31, 2022 at 11:07:58AM -0700, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@intel.com>
> 
> Kernels built with CONFIG_PREEMPTION=n and CONFIG_PREEMPT_COUNT=y maintain
> preempt_count() state.  Because such kernels map __rcu_read_lock()
> and __rcu_read_unlock() to preempt_disable() and preempt_enable(),
> respectively, this allows the expedited grace period's !CONFIG_PREEMPT_RCU
> version of the rcu_exp_handler() IPI handler function to use
> preempt_count() to detect quiescent states.
> 
> This preempt_count() usage might seem to risk failures due to
> use of implicit RCU readers in portions of the kernel under #ifndef
> CONFIG_PREEMPTION, except that rcu_core() already disallows such implicit
> RCU readers.  The moral of this story is that you must use explicit
> read-side markings such as rcu_read_lock() or preempt_disable() even if
> the code knows that this kernel does not support preemption.
> 
> This commit therefore adds a preempt_count()-based check for a quiescent
> state in the !CONFIG_PREEMPT_RCU version of the rcu_exp_handler()
> function for kernels built with CONFIG_PREEMPT_COUNT=y, reporting an
> immediate quiescent state when the interrupted code had both preemption
> and softirqs enabled.
> 
> This change results in about a 2% reduction in expedited grace-period
> latency in kernels built with both CONFIG_PREEMPT_RCU=n and
> CONFIG_PREEMPT_COUNT=y.
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Link: https://lore.kernel.org/all/20220622103549.2840087-1-qiang1.zhang@intel.com/
> ---
>  kernel/rcu/tree_exp.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index be667583a5547..b07998159d1fa 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -828,11 +828,13 @@ static void rcu_exp_handler(void *unused)
>  {
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  	struct rcu_node *rnp = rdp->mynode;
> +	bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK |
> SOFTIRQ_MASK));

I don't know if nested hardirqs still exist. I only heard old rumours
about broken drivers. Should we take care of them?

Also are we sure that all callers of flush_smp_call_function_queue()
are QS?

Let's see we know that rcu_exp_handler() can either be executed from:

* hardirqs

Or from process context, expected to be RCU QS states at least in idle
as the comment above flush_smp_call_function_queue() in idle says
(but I'd rather check all the in-process callers before stating all
of them are in QS)

* idle (in which case preemption is disabled unfortunately so the current
test won't help)
* stop_machine
  _ When CPU is dead and out of RCU (rcutree_dead_cpu() called)
    so that should be a QS.
  _ When CPU is migrating (is it a QS then?)

If we check further that all non-IRQ callers of flush_smp_call_function_queue()
are always quiescent states then we could deduce that !in_hardirq() means we are in
a quiescent state, whether preemption is disabled or not.

In any case for the current patch, perhaps a more robust test against nested
hardirqs would be:

unsigned long cnt = preempt_count();
bool preempt_bh_enabled = (!cnt || cnt == HARDIRQ_OFFSET)

Thanks.

>  
>  	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
>  	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
>  		return;
> -	if (rcu_is_cpu_rrupt_from_idle()) {
> +	if (rcu_is_cpu_rrupt_from_idle() ||
> +	    (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preempt_bh_enabled)) {
>  		rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
>  		return;
>  	}
> -- 
> 2.31.1.189.g2e36527f23
>