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 :)
> 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.
> Thanx, Paul
Sebastian
