On Fri, Jun 07, 2019 at 06:08:57PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-05-28 13:50:30 [-0700], Paul E. McKenney wrote:
> > Hello, Sebastian,
> Hi Paul,
>
> > Finally getting around to taking another look:
> >
> > c7e07056a108 EXP rcu: skip the workqueue path on RT
> >
> > This one makes sense given the later commit setting the
> > rcu_normal_after_boot kernel parameter. Otherwise, it is
> > slowing down expedited grace periods for no reason. But
> > should the check also include rcu_normal_after_boot and
> > rcu_normal? For example:
> >
> > if ((IS_ENABLED(CONFIG_PREEMPT_RT_FULL) &&
> > (rcu_normal || rcu_normal_after_boot) ||
> > !READ_ONCE(rcu_par_gp_wq) ||
> > rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
> > rcu_is_last_leaf_node(rnp)) {
>
> I recently dropped that patch from the queue because the workqueue
> problem vanished.
>
> > Alternatively, one approach would be to take the kernel
> > parameters out in -rt:
> >
> > static int rcu_normal_after_boot =
> > IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> > #ifndef CONFIG_PREEMPT_RT_FULL
> > module_param(rcu_normal_after_boot, int, 0);
> > #endif
> >
> > And similar for rcu_normal and rcu_expedited.
>
> This makes sense.
>
> > Or is there some reason to allow run-time expedited grace
> > periods in CONFIG_PREEMPT_RT_FULL=y kernels?
>
> No, I doubt there is any need to use the `expedited' version. The
> problem is that it increases latencies.
>
> > d1f52391bd8a rcu: Disable RCU_FAST_NO_HZ on RT
> >
> > Looks good. More complexity could be added if too many people
> > get themselves in trouble via "select RCU_FAST_NO_HZ".
>
> That patch disables RCU_FAST_NO_HZ and claims that it has something to
> do with a timer_list timer and IRQ-off section. We couldn't schedule
> timers from IRQ-off regions but not anymore. Only del_timer_sync() can't
> be invoked from IRQ-off regions.
> I just booted a box with this enabled together with NO_HZ/ NO_HZ_FULL
> and I not complains yet. So I might drop that…
>
> > 42b346870326 rcu: make RCU_BOOST default on RT
> >
> > To avoid complaints about this showing up when people don't
> > expected, could you please instead "select RCU_BOOST" in
> > the Kconfig definition of PREEMPT_RT_FULL?
> >
> > Or do people really want to be able to disable boosting?
>
> I have no idea. I guess most people don't know what it does and stay
> with the default. It become default on RT once a few people complained
> that they run OOM during boot on some "memory contrained systems". That
> option avoided it.
> So yes, will make it depend on RT.
>
> > 457c1b0d9c0e sched: Do not account rcu_preempt_depth on RT in might_sleep()
> >
> > The idea behind this one is to avoid false-positive complaints
> > about -rt's sleeping spinlocks, correct?
>
> Correct. Maybe we could invoke a different schedule() primitiv so RCU is
> aware that this is a sleeping spinlock and not a regular sleeping lock.
>
> > 7ee13e640b01 rbtree: don't include the rcu header
> > c9b0c9b87081 rtmutex: annotate sleeping lock context
> >
> > No specific comments.
> >
> > 7912d002ebf9 rcu: Eliminate softirq processing from rcutree
> >
> > This hasn't caused any problems in -rcu from what I can see.
> > I am therefore planning to submit the -rcu variant of this to
> > mainline during the next merge window.
>
> wonderful.
>
> > f06d34ebdbbb srcu: Remove srcu_queue_delayed_work_on()
> >
> > Looks plausible. I will check more carefully for mainline.
>
> Hmmm. I though this was already upstream.
> That said, we can now schedule work from a preempt_disable() section but
> I still like the negative diffstat here :)
Right you are! e81baf4cb19a ("srcu: Remove srcu_queue_delayed_work_on()")
is in v5.1.
> > aeb04e894cc9 srcu: replace local_irqsave() with a locallock
> > e48989b033ad irqwork: push most work into softirq context
> >
> > These look to still be -rt only.
>
> I might get rid of the local_lock in srcu. Will have to check.
>
> Thank you Paul.
And you! I will check again in a few months, for some definition of "a few".
Thanx, Paul
