On Wed, Aug 31, 2022 at 11:07:58AM -0700, Paul E. McKenney wrote: > From: Zqiang <qiang1.zhang@xxxxxxxxx> > > 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@xxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > Link: https://lore.kernel.org/all/20220622103549.2840087-1-qiang1.zhang@xxxxxxxxx/ > --- > 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 >
