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 :) -- Vlad Rezki