On Wed, Sep 07, 2022 at 07:57:59AM -0700, Paul E. McKenney wrote: > On Wed, Sep 07, 2022 at 02:10:10PM +0200, Frederic Weisbecker wrote: > > 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? > > Last I checked, certain tracing scenarios from irq handlers looked > to RCU like nested irq handlers. Given that, does your more robust > approach below work correctly? I haven't observed that but in any case, the check I propose is more strict than the one on this patch. So in the worst case it's a QS not reported if a nested interrupt is detected. Thanks.
