Re: [PATCH 19/24] rcu/tree: Support reclaim for head-less object

Joel Fernandes
2020-05-04T00:32:41Z

On Sun, May 03, 2020 at 05:28:55PM -0700, Paul E. McKenney wrote:
> On Sun, May 03, 2020 at 08:12:58PM -0400, Joel Fernandes wrote:
> > On Fri, May 01, 2020 at 03:39:09PM -0700, Paul E. McKenney wrote:
> > > On Tue, Apr 28, 2020 at 10:58:58PM +0200, Uladzislau Rezki (Sony) wrote:
> > > > Update the kvfree_call_rcu() with head-less support, it
> > > > means an object without any rcu_head structure can be
> > > > reclaimed after GP.
> > > >
> > > > To store pointers there are two chain-arrays maintained
> > > > one for SLAB and another one is for vmalloc. Both types
> > > > of objects(head-less variant and regular one) are placed
> > > > there based on the type.
> > > >
> > > > It can be that maintaining of arrays becomes impossible
> > > > due to high memory pressure. For such reason there is an
> > > > emergency path. In that case objects with rcu_head inside
> > > > are just queued building one way list. Later on that list
> > > > is drained.
> > > >
> > > > As for head-less variant. Such objects do not have any
> > > > rcu_head helper inside. Thus it is dynamically attached.
> > > > As a result an object consists of back-pointer and regular
> > > > rcu_head. It implies that emergency path can detect such
> > > > object type, therefore they are tagged. So a back-pointer
> > > > could be freed as well as dynamically attached wrapper.
> > > >
> > > > Even though such approach requires dynamic memory it needs
> > > > only sizeof(unsigned long *) + sizeof(struct rcu_head) bytes,
> > > > thus SLAB is used to obtain it. Finally if attaching of the
> > > > rcu_head and queuing get failed, the current context has
> > > > to follow might_sleep() annotation, thus below steps could
> > > > be applied:
> > > > a) wait until a grace period has elapsed;
> > > > b) direct inlining of the kvfree() call.
> > > >
> > > > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > ---
> > > > kernel/rcu/tree.c | 102 ++++++++++++++++++++++++++++++++++++++++++++--
> > > > 1 file changed, 98 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 51726e4c3b4d..501cac02146d 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3072,15 +3072,31 @@ static void kfree_rcu_work(struct work_struct *work)
> > > > */
> > > > for (; head; head = next) {
> > > > unsigned long offset = (unsigned long)head->func;
> > > > - void *ptr = (void *)head - offset;
> > > > + bool headless;
> > > > + void *ptr;
> > > >
> > > > next = head->next;
> > > > +
> > > > + /* We tag the headless object, if so adjust offset. */
> > > > + headless = (((unsigned long) head - offset) & BIT(0));
> > > > + if (headless)
> > > > + offset -= 1;
> > > > +
> > > > + ptr = (void *) head - offset;
> > > > +
> > > > debug_rcu_head_unqueue((struct rcu_head *)ptr);
> > > > rcu_lock_acquire(&rcu_callback_map);
> > > > trace_rcu_invoke_kvfree_callback(rcu_state.name, head, offset);
> > > >
> > > > - if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset)))
> > > > + if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset))) {
> > > > + /*
> > > > + * If headless free the back-pointer first.
> > > > + */
> > > > + if (headless)
> > > > + kvfree((void *) *((unsigned long *) ptr));
> > > > +
> > > > kvfree(ptr);
> > > > + }
> > > >
> > > > rcu_lock_release(&rcu_callback_map);
> > > > cond_resched_tasks_rcu_qs();
> > > > @@ -3221,6 +3237,13 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > > > if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > > > return false;
> > > >
> > > > + /*
> > > > + * TODO: For one argument of kvfree_rcu() we can
> > > > + * drop the lock and get the page in sleepable
> > > > + * context. That would allow to maintain an array
> > > > + * for the CONFIG_PREEMPT_RT as well. Thus we could
> > > > + * get rid of dynamic rcu_head attaching code.
> > > > + */
> > > > bnode = (struct kvfree_rcu_bulk_data *)
> > > > __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > > > }
> > > > @@ -3244,6 +3267,23 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > > > return true;
> > > > }
> > > >
> > > > +static inline struct rcu_head *
> > > > +attach_rcu_head_to_object(void *obj)
> > > > +{
> > > > + unsigned long *ptr;
> > > > +
> > > > + ptr = kmalloc(sizeof(unsigned long *) +
> > > > + sizeof(struct rcu_head), GFP_NOWAIT |
> > > > + __GFP_RECLAIM | /* can do direct reclaim. */
> > > > + __GFP_NORETRY | /* only lightweight one. */
> > > > + __GFP_NOWARN); /* no failure reports. */
> > >
> > > Again, let's please not do this single-pointer-sized allocation. If
> > > a full page is not available and this is a single-argument kfree_rcu(),
> > > just call synchronize_rcu() and then free the object directly.
> >
> > With the additional caching, lack of full page should not be very likely. I
> > agree we can avoid doing any allocation and just straight to
> > synchroize_rcu().
>
> That sounds good to me!
>
> > > It should not be -that- hard to adjust locking for CONFIG_PREEMPT_RT!
> > > For example, have some kind of reservation protocol so that a task
> > > that drops the lock can retry the page allocation and be sure of having
> > > a place to put it. This might entail making CONFIG_PREEMPT_RT reserve
> > > more pages per CPU. Or maybe that would not be necessary.
> >
> > If we are not doing single-pointer allocation, then that would also eliminate
> > entering the low-level page allocator for single-pointer allocations.
> >
> > Or did you mean entry into the allocator for the full-page allocations
> > related to the pointer array for PREEMPT_RT? Even if we skip entry into the
> > allocator for those, we will still have additional caching which further
> > reduces chances of getting a full page. In the event of such failure, we can
> > simply queue the rcu_head.
> >
> > Thoughts?
>
> I was just trying to guess why you kept the single-pointer allocation.
> It looks like I guessed wrong. ;-)
>
> If, as you say above, you make it go straight to synchronize_rcu()
> upon full-page allocation failure, that would be good!

Paul, sounds good. Vlad, are you also Ok with that?

thanks,

- Joel

> Thanx, Paul
>
> > thanks,
> >
> > - Joel
> >
> > >
> > > Thanx, Paul
> > >
> > > > + if (!ptr)
> > > > + return NULL;
> > > > +
> > > > + ptr[0] = (unsigned long) obj;
> > > > + return ((struct rcu_head *) ++ptr);
> > > > +}
> > > > +
> > > > /*
> > > > * Queue a request for lazy invocation of appropriate free routine after a
> > > > * grace period. Please note there are three paths are maintained, two are the
> > > > @@ -3260,16 +3300,34 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > > {
> > > > unsigned long flags;
> > > > struct kfree_rcu_cpu *krcp;
> > > > + bool success;
> > > > void *ptr;
> > > >
> > > > + if (head) {
> > > > + ptr = (void *) head - (unsigned long) func;
> > > > + } else {
> > > > + /*
> > > > + * Please note there is a limitation for the head-less
> > > > + * variant, that is why there is a clear rule for such
> > > > + * objects:
> > > > + *
> > > > + * it can be used from might_sleep() context only. For
> > > > + * other places please embed an rcu_head to your data.
> > > > + */
> > > > + might_sleep();
> > > > + ptr = (unsigned long *) func;
> > > > + }
> > > > +
> > > > krcp = krc_this_cpu_lock(&flags);
> > > > - ptr = (void *)head - (unsigned long)func;
> > > >
> > > > /* Queue the object but don't yet schedule the batch. */
> > > > if (debug_rcu_head_queue(ptr)) {
> > > > /* Probable double kfree_rcu(), just leak. */
> > > > WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
> > > > __func__, head);
> > > > +
> > > > + /* Mark as success and leave. */
> > > > + success = true;
> > > > goto unlock_return;
> > > > }
> > > >
> > > > @@ -3277,10 +3335,34 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > > * Under high memory pressure GFP_NOWAIT can fail,
> > > > * in that case the emergency path is maintained.
> > > > */
> > > > - if (unlikely(!kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr))) {
> > > > + success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
> > > > + if (!success) {
> > > > + if (head == NULL) {
> > > > + /*
> > > > + * Headless(one argument kvfree_rcu()) can sleep.
> > > > + * Drop the lock and tack it back. So it can do
> > > > + * direct lightweight reclaim.
> > > > + */
> > > > + krc_this_cpu_unlock(krcp, flags);
> > > > + head = attach_rcu_head_to_object(ptr);
> > > > + krcp = krc_this_cpu_lock(&flags);
> > > > +
> > > > + if (head == NULL)
> > > > + goto unlock_return;
> > > > +
> > > > + /*
> > > > + * Tag the headless object. Such objects have a
> > > > + * back-pointer to the original allocated memory,
> > > > + * that has to be freed as well as dynamically
> > > > + * attached wrapper/head.
> > > > + */
> > > > + func = (rcu_callback_t) (sizeof(unsigned long *) + 1);
> > > > + }
> > > > +
> > > > head->func = func;
> > > > head->next = krcp->head;
> > > > krcp->head = head;
> > > > + success = true;
> > > > }
> > > >
> > > > WRITE_ONCE(krcp->count, krcp->count + 1);
> > > > @@ -3294,6 +3376,18 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > >
> > > > unlock_return:
> > > > krc_this_cpu_unlock(krcp, flags);
> > > > +
> > > > + /*
> > > > + * High memory pressure, so inline kvfree() after
> > > > + * synchronize_rcu(). We can do it from might_sleep()
> > > > + * context only, so the current CPU can pass the QS
> > > > + * state.
> > > > + */
> > > > + if (!success) {
> > > > + debug_rcu_head_unqueue(ptr);
> > > > + synchronize_rcu();
> > > > + kvfree(ptr);
> > > > + }
> > > > }
> > > > EXPORT_SYMBOL_GPL(kvfree_call_rcu);
> > > >
> > > > --
> > > > 2.20.1
> > > >