On Mon, Apr 20, 2020 at 10:17:23PM +0200, Uladzislau Rezki wrote: > On Mon, Apr 20, 2020 at 09:06:50PM +0200, Uladzislau Rezki wrote: > > On Mon, Apr 20, 2020 at 10:59:15AM -0700, Paul E. McKenney wrote: > > > On Mon, Apr 20, 2020 at 07:40:19PM +0200, Uladzislau Rezki wrote: > > > > On Mon, Apr 20, 2020 at 10:21:26AM -0700, Paul E. McKenney wrote: > > > > > On Mon, Apr 20, 2020 at 06:59:24PM +0200, Uladzislau Rezki wrote: > > > > > > On Mon, Apr 20, 2020 at 09:46:57AM -0700, Paul E. McKenney wrote: > > > > > > > On Mon, Apr 20, 2020 at 06:29:00PM +0200, Uladzislau Rezki wrote: > > > > > > > > On Mon, Apr 20, 2020 at 09:25:34AM -0700, Paul E. McKenney wrote: > > > > > > > > > On Mon, Apr 20, 2020 at 06:08:47PM +0200, Uladzislau Rezki wrote: > > > > > > > > > > On Mon, Apr 20, 2020 at 06:26:01AM -0700, Paul E. McKenney wrote: > > > > > > > > > > > On Mon, Apr 20, 2020 at 03:00:03PM +0200, Uladzislau Rezki wrote: > > > > > > > > > > > > On Mon, Apr 20, 2020 at 08:36:31AM -0400, joel@xxxxxxxxxxxxxxxxx wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On April 20, 2020 8:13:16 AM EDT, Uladzislau Rezki <urezki@xxxxxxxxx> wrote: > > > > > > > > > > > > > >On Sun, Apr 19, 2020 at 06:44:50PM -0700, Paul E. McKenney wrote: > > > > > > > > > > > > > >> On Sun, Apr 19, 2020 at 09:17:49PM -0400, Joel Fernandes wrote: > > > > > > > > > > > > > >> > On Sun, Apr 19, 2020 at 08:27:13PM -0400, Joel Fernandes wrote: > > > > > > > > > > > > > >> > > On Sun, Apr 19, 2020 at 07:58:36AM -0700, Paul E. McKenney wrote: > > > > > > > > > > > > > >> > > > On Sat, Apr 18, 2020 at 02:37:48PM +0200, Uladzislau Rezki > > > > > > > > > > > > > >wrote: > > > > > > > > > > > > > >> > > > > On Fri, Apr 17, 2020 at 11:54:49AM -0700, Paul E. McKenney > > > > > > > > > > > > > >wrote: > > > > > > > > > > > > > >> > > > > > On Fri, Apr 17, 2020 at 02:26:41PM -0400, Joel Fernandes > > > > > > > > > > > > > >wrote: > > > > > > > > > > > > > >> > > > > > > On Fri, Apr 17, 2020 at 05:04:42PM +0200, Sebastian > > > > > > > > > > > > > >Andrzej Siewior wrote: > > > > > > > > > > > > > >> > > > > > > > On 2020-04-16 23:05:15 [-0400], Joel Fernandes wrote: > > > > > > > > > > > > > >> > > > > > > > > On Thu, Apr 16, 2020 at 11:34:44PM +0200, Sebastian > > > > > > > > > > > > > >Andrzej Siewior wrote: > > > > > > > > > > > > > >> > > > > > > > > > On 2020-04-16 14:00:57 [-0700], Paul E. McKenney > > > > > > > > > > > > > >wrote: > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > >> > > > > > > > > > > We might need different calling-context > > > > > > > > > > > > > >restrictions for the two variants > > > > > > > > > > > > > >> > > > > > > > > > > of kfree_rcu(). And we might need to come up > > > > > > > > > > > > > >with some sort of lockdep > > > > > > > > > > > > > >> > > > > > > > > > > check for "safe to use normal spinlock in -rt". > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > >> > > > > > > > > > Oh. We do have this already, it is called > > > > > > > > > > > > > >CONFIG_PROVE_RAW_LOCK_NESTING. > > > > > > > > > > > > > >> > > > > > > > > > This one will scream if you do > > > > > > > > > > > > > >> > > > > > > > > > raw_spin_lock(); > > > > > > > > > > > > > >> > > > > > > > > > spin_lock(); > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > >> > > > > > > > > > Sadly, as of today, there is code triggering this > > > > > > > > > > > > > >which needs to be > > > > > > > > > > > > > >> > > > > > > > > > addressed first (but it is one list of things to > > > > > > > > > > > > > >do). > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > >> > > > > > > > > > Given the thread so far, is it okay if I repost the > > > > > > > > > > > > > >series with > > > > > > > > > > > > > >> > > > > > > > > > migrate_disable() instead of accepting a possible > > > > > > > > > > > > > >migration before > > > > > > > > > > > > > >> > > > > > > > > > grabbing the lock? I would prefer to avoid the > > > > > > > > > > > > > >extra RT case (avoiding > > > > > > > > > > > > > >> > > > > > > > > > memory allocations in a possible atomic context) > > > > > > > > > > > > > >until we get there. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > >> > > > > > > > > I prefer something like the following to make it > > > > > > > > > > > > > >possible to invoke > > > > > > > > > > > > > >> > > > > > > > > kfree_rcu() from atomic context considering > > > > > > > > > > > > > >call_rcu() is already callable > > > > > > > > > > > > > >> > > > > > > > > from such contexts. Thoughts? > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > >> > > > > > > > So it looks like it would work. However, could we > > > > > > > > > > > > > >please delay this > > > > > > > > > > > > > >> > > > > > > > until we have an actual case on RT? I just added > > > > > > > > > > > > > >> > > > > > > > WARN_ON(!preemptible()); > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > > > > > I am not sure if waiting for it to break in the future is > > > > > > > > > > > > > >a good idea. I'd > > > > > > > > > > > > > >> > > > > > > rather design it in a forward thinking way. There could > > > > > > > > > > > > > >be folks replacing > > > > > > > > > > > > > >> > > > > > > "call_rcu() + kfree in a callback" with kfree_rcu() for > > > > > > > > > > > > > >example. If they were > > > > > > > > > > > > > >> > > > > > > in !preemptible(), we'd break on page allocation. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > > > > > Also as a sidenote, the additional pre-allocation of > > > > > > > > > > > > > >pages that Vlad is > > > > > > > > > > > > > >> > > > > > > planning on adding would further reduce the need for > > > > > > > > > > > > > >pages from the page > > > > > > > > > > > > > >> > > > > > > allocator. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > > > > > Paul, what is your opinion on this? > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > > > > My experience with call_rcu(), of which kfree_rcu() is a > > > > > > > > > > > > > >specialization, > > > > > > > > > > > > > >> > > > > > is that it gets invoked with preemption disabled, with > > > > > > > > > > > > > >interrupts > > > > > > > > > > > > > >> > > > > > disabled, and during early boot, as in even before > > > > > > > > > > > > > >rcu_init() has been > > > > > > > > > > > > > >> > > > > > invoked. This experience does make me lean towards raw > > > > > > > > > > > > > >spinlocks. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > > > > But to Sebastian's point, if we are going to use raw > > > > > > > > > > > > > >spinlocks, we need > > > > > > > > > > > > > >> > > > > > to keep the code paths holding those spinlocks as short as > > > > > > > > > > > > > >possible. > > > > > > > > > > > > > >> > > > > > I suppose that the inability to allocate memory with raw > > > > > > > > > > > > > >spinlocks held > > > > > > > > > > > > > >> > > > > > helps, but it is worth checking. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > > > How about reducing the lock contention even further? > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > Can we do even better by moving the work-scheduling out from > > > > > > > > > > > > > >under the > > > > > > > > > > > > > >> > > > spinlock? This of course means that it is necessary to handle > > > > > > > > > > > > > >the > > > > > > > > > > > > > >> > > > occasional spurious call to the work handler, but that should > > > > > > > > > > > > > >be rare > > > > > > > > > > > > > >> > > > and should be in the noise compared to the reduction in > > > > > > > > > > > > > >contention. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > Yes I think that will be required since -rt will sleep on > > > > > > > > > > > > > >workqueue locks as > > > > > > > > > > > > > >> > > well :-(. I'm looking into it right now. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > /* > > > > > > > > > > > > > >> > > * If @work was previously on a different pool, it might > > > > > > > > > > > > > >still be > > > > > > > > > > > > > >> > > * running there, in which case the work needs to be > > > > > > > > > > > > > >queued on that > > > > > > > > > > > > > >> > > * pool to guarantee non-reentrancy. > > > > > > > > > > > > > >> > > */ > > > > > > > > > > > > > >> > > last_pool = get_work_pool(work); > > > > > > > > > > > > > >> > > if (last_pool && last_pool != pwq->pool) { > > > > > > > > > > > > > >> > > struct worker *worker; > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > spin_lock(&last_pool->lock); > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > Hmm, I think moving schedule_delayed_work() outside lock will work. > > > > > > > > > > > > > >Just took > > > > > > > > > > > > > >> > a good look and that's not an issue. However calling > > > > > > > > > > > > > >schedule_delayed_work() > > > > > > > > > > > > > >> > itself is an issue if the caller of kfree_rcu() is !preemptible() > > > > > > > > > > > > > >on > > > > > > > > > > > > > >> > PREEMPT_RT. Because the schedule_delayed_work() calls spin_lock on > > > > > > > > > > > > > >pool->lock > > > > > > > > > > > > > >> > which can sleep on PREEMPT_RT :-(. Which means we have to do either > > > > > > > > > > > > > >of: > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > 1. Implement a new mechanism for scheduling delayed work that does > > > > > > > > > > > > > >not > > > > > > > > > > > > > >> > acquire sleeping locks. > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > 2. Allow kfree_rcu() only from preemptible context (That is > > > > > > > > > > > > > >Sebastian's > > > > > > > > > > > > > >> > initial patch to replace local_irq_save() + spin_lock() with > > > > > > > > > > > > > >> > spin_lock_irqsave()). > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > 3. Queue the work through irq_work or another bottom-half > > > > > > > > > > > > > >mechanism. > > > > > > > > > > > > > >> > > > > > > > > > > > > > >> I use irq_work elsewhere in RCU, but the queue_delayed_work() might > > > > > > > > > > > > > >> go well with a timer. This can of course be done conditionally. > > > > > > > > > > > > > >> > > > > > > > > > > > > > >We can schedule_delayed_work() inside and outside of the spinlock, > > > > > > > > > > > > > >i.e. it is not an issue for RT kernel, because as it was noted in last > > > > > > > > > > > > > >message a workqueue system uses raw spinlicks internally. I checked > > > > > > > > > > > > > >the latest linux-5.6.y-rt also. If we do it inside, we will place the > > > > > > > > > > > > > >work on current CPU, at least as i see it, even if it is "unbound". > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks for confirming!! > > > > > > > > > > > > > > > > > > > > > > > > > > >If we do it outside, we will reduce a critical section, from the other > > > > > > > > > > > > > >hand we can introduce a potential delay in placing the context into > > > > > > > > > > > > > >CPUs > > > > > > > > > > > > > >run-queuye. As a result we could end up on another CPU, thus placing > > > > > > > > > > > > > >the work on new CPU, plus memory foot-print might be higher. It would > > > > > > > > > > > > > >be good to test and have a look at it actually. > > > > > > > > > > > > > > > > > > > > > > > > > > > >But it can be negligible :) > > > > > > > > > > > > > > > > > > > > > > > > > > Since the wq locking is raw spinlock on rt as Mike and you mentioned, if wq holds lock for too long that itself will spawn a lengthy non preemptible critical section, so from that standpoint doing it under our lock should be ok I think. > > > > > > > > > > > > > > > > > > > > > > > > > It should be OK, i do not expect to get noticeable latency for any RT > > > > > > > > > > > > workloads. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >> > Any other thoughts? > > > > > > > > > > > > > >> > > > > > > > > > > > > > >> I did forget to ask you guys your opinions about the downsides (if > > > > > > > > > > > > > >any) > > > > > > > > > > > > > >> of moving from unbound to per-CPU workqueues. Thoughts? > > > > > > > > > > > > > >> > > > > > > > > > > > > > >If we do it outside of spinlock, there is at least one drawback that i > > > > > > > > > > > > > >see, i described it above. We can use schedule_delayed_work_on() but > > > > > > > > > > > > > >we as a caller have to guarantee that a CPU we about to place a work > > > > > > > > > > > > > >is alive :) > > > > > > > > > > > > > > > > > > > > > > > > > > FWIW, some time back I did a simple manual test calling queue_work_on on an offline CPU to see what happens and it appears to be working fine. On a 4 CPU system, I offline CPU 3 and queue the work on it which ends up executing on CPU 0 instead. > > > > > > > > > > > > > > > > > > > > > > > > > <snip> > > > > > > > > > > > > /** > > > > > > > > > > > > * queue_work_on - queue work on specific cpu > > > > > > > > > > > > * @cpu: CPU number to execute work on > > > > > > > > > > > > * @wq: workqueue to use > > > > > > > > > > > > * @work: work to queue > > > > > > > > > > > > * > > > > > > > > > > > > * We queue the work to a specific CPU, the caller must ensure it > > > > > > > > > > > > * can't go away. > > > > > > > > > > > > * > > > > > > > > > > > > * Return: %false if @work was already on a queue, %true otherwise. > > > > > > > > > > > > */ > > > > > > > > > > > > <snip> > > > > > > > > > > > > > > > > > > > > > > > > It says, how i see it, we should ensure it can not go away. So, if > > > > > > > > > > > > we drop the lock we should do like: > > > > > > > > > > > > > > > > > > > > > > > > get_online_cpus(); > > > > > > > > > > > > check a CPU is onlen; > > > > > > > > > > > > queue_work_on(); > > > > > > > > > > > > put_online_cpus(); > > > > > > > > > > > > > > > > > > > > > > > > but i suspect we do not want to do it :) > > > > > > > > > > > > > > > > > > > > > > Indeed, it might impose a few restrictions and a bit of overhead that > > > > > > > > > > > might not be welcome at some point in the future. ;-) > > > > > > > > > > > > > > > > > > > > > > On top of this there are potential load-balancing concerns. By specifying > > > > > > > > > > > the CPU, you are limiting workqueue's and scheduler's ability to adjust to > > > > > > > > > > > any sudden changes in load. Maybe not enough to matter in most cases, but > > > > > > > > > > > might be an issue if there is a sudden flood of kfree_rcu() invocations. > > > > > > > > > > > > > > > > > > > > > Agree. Let's keep it as it is now :) > > > > > > > > > > > > > > > > > > I am not sure which "as it is now" you are referring to, but I suspect > > > > > > > > > that the -rt guys prefer two short interrupts-disabled regions to one > > > > > > > > > longer interrupts-disabled region. > > > > > > > > > > > > > > > > I mean to run schedule_delayed_work() under spinlock. > > > > > > > > > > > > > > Which is an interrupt-disabled spinlock, correct? > > > > > > > > > > > > > To do it under holding the lock, currently it is spinlock, but it is > > > > > > going to be(if you agree :)) raw ones, which keeps IRQs disabled. I > > > > > > saw Joel sent out patches. > > > > > > > > > > Then please move the schedule_delayed_work() and friends out from > > > > > under the spinlock. Unless Sebastian has some reason why extending > > > > > an interrupts-disabled critical section (and thus degrading real-time > > > > > latency) is somehow OK in this case. > > > > > > > > > Paul, if move outside of the lock we may introduce unneeded migration > > > > issues, plus it can introduce higher memory footprint(i have not tested). > > > > I have described it in more detail earlier in this mail thread. I do not > > > > think that waking up the work is an issue for RT from latency point of > > > > view. But let's ask Sebastian to confirm. > > > > > > > > Sebastian, do you think that placing a work on current CPU is an issue? > > > > If we do it under raw spinlock? > > > > > > We really are talking past each other, aren't we? ;-) > > > > > Let's hear each other better then :) > > > > > > > > My concern is lengthening the duration of the critical section by having > > > the extra work-queuing execution within it. As in leave the workqueue > > > free to migrate, but invoke it after releasing the lock. > > > > Paul, i have just measured the time duration of the schedule_delayed_work(). > To do that i used below patch: > > <snip> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 02f73f7bbd40..f74ae0f3556e 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3232,6 +3232,12 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj) > return ((struct rcu_head *) ++ptr); > } > > +static void noinline > +measure_schedule_delayed_work(struct kfree_rcu_cpu *krcp) > +{ > + schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES); > +} > + > /* > * Queue a request for lazy invocation of appropriate free routine after a > * grace period. Please note there are three paths are maintained, two are the > @@ -3327,8 +3333,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) > if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING && > !krcp->monitor_todo) { > krcp->monitor_todo = true; > - schedule_delayed_work(&krcp->monitor_work, > - expedited_drain ? 0 : KFREE_DRAIN_JIFFIES); > + measure_schedule_delayed_work(krcp); > } > <snip> > > i have done it for not CONFIG_PREEMPT_RT kernel, i do not have any RT configuration. > I run rcuperf to apply the load to see the time taken by the actual placing of the work, > i.e. the time taken by schedule_delayed_work(): Thank you for measuring this! We do need Sebastian's eyes on this. I am not sure what the latency goals are now, but in the past they have been in the range of a few tens of microseconds total latency, in which case a five-microsecond change can make a difference. To say nothing of the ten-microsecond latency in one of the measurements. Thanx, Paul > <snip> > root@pc636:/sys/kernel/debug/tracing# cat trace > # tracer: function_graph > # > # function_graph latency trace v1.1.5 on 5.6.0-rc6+ > # -------------------------------------------------------------------- > # latency: 0 us, #16/16, CPU#0 | (M:server VP:0, KP:0, SP:0 HP:0 #P:4) > # ----------------- > # | task: -0 (uid:0 nice:0 policy:0 rt_prio:0) > # ----------------- > # > # _-----=> irqs-off > # / _----=> need-resched > # | / _---=> hardirq/softirq > # || / _--=> preempt-depth > # ||| / > # TIME CPU TASK/PID |||| DURATION FUNCTION CALLS > # | | | | |||| | | | | | | > 682.384653 | 1) <idle>-0 | d.s. | 5.329 us | } /* measure_schedule_delayed_work.constprop.86 */ > 685.374654 | 2) <idle>-0 | d.s. | 5.392 us | } /* measure_schedule_delayed_work.constprop.86 */ > 700.304647 | 2) <idle>-0 | d.s. | 5.650 us | } /* measure_schedule_delayed_work.constprop.86 */ > 710.331280 | 3) <idle>-0 | d.s. | 5.145 us | } /* measure_schedule_delayed_work.constprop.86 */ > 714.387943 | 1) <idle>-0 | d.s. | 9.986 us | } /* measure_schedule_delayed_work.constprop.86 */ > 720.251229 | 0) <idle>-0 | d.s. | 5.292 us | } /* measure_schedule_delayed_work.constprop.86 */ > 725.211208 | 2) <idle>-0 | d.s. | 5.295 us | } /* measure_schedule_delayed_work.constprop.86 */ > 731.847845 | 1) <idle>-0 | d.s. | 5.048 us | } /* measure_schedule_delayed_work.constprop.86 */ > 736.357802 | 2) <idle>-0 | d.s. | 5.134 us | } /* measure_schedule_delayed_work.constprop.86 */ > 738.287785 | 1) <idle>-0 | d.s. | 5.863 us | } /* measure_schedule_delayed_work.constprop.86 */ > 742.214431 | 1) <idle>-0 | d.s. | 5.202 us | } /* measure_schedule_delayed_work.constprop.86 */ > 759.844264 | 2) <idle>-0 | d.s. | 5.375 us | } /* measure_schedule_delayed_work.constprop.86 */ > 764.304218 | 1) <idle>-0 | d.s. | 5.650 us | } /* measure_schedule_delayed_work.constprop.86 */ > 766.224204 | 3) <idle>-0 | d.s. | 5.015 us | } /* measure_schedule_delayed_work.constprop.86 */ > 772.410794 | 1) <idle>-0 | d.s. | 5.061 us | } /* measure_schedule_delayed_work.constprop.86 */ > 781.370691 | 1) <idle>-0 | d.s. | 5.165 us | } /* measure_schedule_delayed_work.constprop.86 */ > root@pc636:/sys/kernel/debug/tracing# cat tracing_thresh > 5 > root@pc636:/sys/kernel/debug/tracing# > <snip> > > -- > Vlad Rezki