Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Uladzislau Rezki <urezki@gmail.com>]

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".

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 :)

> > 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 :)

--
Vlad Rezki