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. > >> > 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. Thanks, - Joel > >-- >Vlad Rezki -- Sent from my Android device with K-9 Mail. Please excuse my brevity.