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

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Apr 20, 2020 at 08:36:31AM -0400, joel@joelfernandes.org wrote:
> 
> 
> On April 20, 2020 8:13:16 AM EDT, Uladzislau Rezki <urezki@gmail.com> 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