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

[Uladzislau Rezki <urezki@gmail.com>]

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?

<snip>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f288477ee1c2..fb916e065784 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3053,7 +3053,8 @@ static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,

        // Previous RCU batch still in progress, try again later.
        krcp->monitor_todo = true;
-       schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+       schedule_delayed_work_on(raw_smp_processor_id(),
+               &krcp->monitor_work, KFREE_DRAIN_JIFFIES);
        spin_unlock_irqrestore(&krcp->lock, flags);
 }

@@ -3168,7 +3169,8 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
        if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
            !krcp->monitor_todo) {
                krcp->monitor_todo = true;
-               schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+               schedule_delayed_work_on(raw_smp_processor_id(),
+                       &krcp->monitor_work, KFREE_DRAIN_JIFFIES);
        }

 unlock_return:
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 891ccad5f271..49fcc50469f4 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1723,7 +1723,9 @@ static void rcu_work_rcufn(struct rcu_head *rcu)

        /* read the comment in __queue_work() */
        local_irq_disable();
-       __queue_work(WORK_CPU_UNBOUND, rwork->wq, &rwork->work);
+
+       /* Just for illustration. Can have queue_rcu_work_on(). */
+       __queue_work(raw_smp_processor_id(), rwork->wq, &rwork->work);
        local_irq_enable();
 }
<snip>

Thoughts?

--
Vlad Rezki