Messages in this thread | | | Date | Fri, 7 Jun 2019 18:08:57 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: Review of RCU-related patches in -rt |
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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
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