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.
--
Vlad Rezki
