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