Re: [PATCH 22/24] rcu/tiny: support reclaim for head-less object

Joel Fernandes
2020-05-04T00:27:05Z

On Fri, May 01, 2020 at 04:06:38PM -0700, Paul E. McKenney wrote:
> On Tue, Apr 28, 2020 at 10:59:01PM +0200, Uladzislau Rezki (Sony) wrote:
> > Make a kvfree_call_rcu() function to support head-less
> > freeing. Same as for tree-RCU, for such purpose we store
> > pointers in array. SLAB and vmalloc ptrs. are mixed and
> > coexist together.
> >
> > Under high memory pressure it can be that maintaining of
> > arrays becomes impossible. Objects with an rcu_head are
> > released via call_rcu(). When it comes to the head-less
> > variant, the kvfree() call is directly inlined, i.e. we
> > do the same as for tree-RCU:
> > a) wait until a grace period has elapsed;
> > b) direct inlining of the kvfree() call.
> >
> > Thus the current context has to follow might_sleep()
> > annotation. Also please note that for tiny-RCU any
> > call of synchronize_rcu() is actually a quiescent
> > state, therefore (a) does nothing.
>
> Please, please, please just do synchronize_rcu() followed by kvfree()
> for single-argument kfree_rcu() and friends in Tiny RCU.
>
> Way simpler and probably way faster as well. And given that Tiny RCU
> runs only on uniprocessor systems, the complexity probably is buying
> you very little, if anything.

Agreed.

thanks,

- Joel

>
> Thanx, Paul
>
> > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> > kernel/rcu/tiny.c | 157 +++++++++++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 156 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> > index 508c82faa45c..b1c31a935db9 100644
> > --- a/kernel/rcu/tiny.c
> > +++ b/kernel/rcu/tiny.c
> > @@ -40,6 +40,29 @@ static struct rcu_ctrlblk rcu_ctrlblk = {
> > .curtail = &rcu_ctrlblk.rcucblist,
> > };
> >
> > +/* Can be common with tree-RCU. */
> > +#define KVFREE_DRAIN_JIFFIES (HZ / 50)
> > +
> > +/* Can be common with tree-RCU. */
> > +struct kvfree_rcu_bulk_data {
> > + unsigned long nr_records;
> > + struct kvfree_rcu_bulk_data *next;
> > + void *records[];
> > +};
> > +
> > +/* Can be common with tree-RCU. */
> > +#define KVFREE_BULK_MAX_ENTR \
> > + ((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *))
> > +
> > +static struct kvfree_rcu_bulk_data *kvhead;
> > +static struct kvfree_rcu_bulk_data *kvhead_free;
> > +static struct kvfree_rcu_bulk_data *kvcache;
> > +
> > +static DEFINE_STATIC_KEY_FALSE(rcu_init_done);
> > +static struct delayed_work monitor_work;
> > +static struct rcu_work rcu_work;
> > +static bool monitor_todo;
> > +
> > void rcu_barrier(void)
> > {
> > wait_rcu_gp(call_rcu);
> > @@ -177,9 +200,137 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
> > }
> > EXPORT_SYMBOL_GPL(call_rcu);
> >
> > +static inline bool
> > +kvfree_call_rcu_add_ptr_to_bulk(void *ptr)
> > +{
> > + struct kvfree_rcu_bulk_data *bnode;
> > +
> > + if (!kvhead || kvhead->nr_records == KVFREE_BULK_MAX_ENTR) {
> > + bnode = xchg(&kvcache, NULL);
> > + if (!bnode)
> > + bnode = (struct kvfree_rcu_bulk_data *)
> > + __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > +
> > + if (unlikely(!bnode))
> > + return false;
> > +
> > + /* Initialize the new block. */
> > + bnode->nr_records = 0;
> > + bnode->next = kvhead;
> > +
> > + /* Attach it to the bvhead. */
> > + kvhead = bnode;
> > + }
> > +
> > + /* Done. */
> > + kvhead->records[kvhead->nr_records++] = ptr;
> > + return true;
> > +}
> > +
> > +static void
> > +kvfree_rcu_work(struct work_struct *work)
> > +{
> > + struct kvfree_rcu_bulk_data *kvhead_tofree, *next;
> > + unsigned long flags;
> > + int i;
> > +
> > + local_irq_save(flags);
> > + kvhead_tofree = kvhead_free;
> > + kvhead_free = NULL;
> > + local_irq_restore(flags);
> > +
> > + /* Reclaim process. */
> > + for (; kvhead_tofree; kvhead_tofree = next) {
> > + next = kvhead_tofree->next;
> > +
> > + for (i = 0; i < kvhead_tofree->nr_records; i++) {
> > + debug_rcu_head_unqueue((struct rcu_head *)
> > + kvhead_tofree->records[i]);
> > + kvfree(kvhead_tofree->records[i]);
> > + }
> > +
> > + if (cmpxchg(&kvcache, NULL, kvhead_tofree))
> > + free_page((unsigned long) kvhead_tofree);
> > + }
> > +}
> > +
> > +static inline bool
> > +queue_kvfree_rcu_work(void)
> > +{
> > + /* Check if the free channel is available. */
> > + if (kvhead_free)
> > + return false;
> > +
> > + kvhead_free = kvhead;
> > + kvhead = NULL;
> > +
> > + /*
> > + * Queue the job for memory reclaim after GP.
> > + */
> > + queue_rcu_work(system_wq, &rcu_work);
> > + return true;
> > +}
> > +
> > +static void kvfree_rcu_monitor(struct work_struct *work)
> > +{
> > + unsigned long flags;
> > + bool queued;
> > +
> > + local_irq_save(flags);
> > + queued = queue_kvfree_rcu_work();
> > + if (queued)
> > + /* Success. */
> > + monitor_todo = false;
> > + local_irq_restore(flags);
> > +
> > + /*
> > + * If previous RCU reclaim process is still in progress,
> > + * schedule the work one more time to try again later.
> > + */
> > + if (monitor_todo)
> > + schedule_delayed_work(&monitor_work,
> > + KVFREE_DRAIN_JIFFIES);
> > +}
> > +
> > void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > {
> > - call_rcu(head, func);
> > + unsigned long flags;
> > + bool success;
> > + void *ptr;
> > +
> > + if (head) {
> > + ptr = (void *) head - (unsigned long) func;
> > + } else {
> > + might_sleep();
> > + ptr = (void *) func;
> > + }
> > +
> > + if (debug_rcu_head_queue(ptr)) {
> > + /* Probable double free, just leak. */
> > + WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
> > + __func__, head);
> > + return;
> > + }
> > +
> > + local_irq_save(flags);
> > + success = kvfree_call_rcu_add_ptr_to_bulk(ptr);
> > + if (static_branch_likely(&rcu_init_done)) {
> > + if (success && !monitor_todo) {
> > + monitor_todo = true;
> > + schedule_delayed_work(&monitor_work,
> > + KVFREE_DRAIN_JIFFIES);
> > + }
> > + }
> > + local_irq_restore(flags);
> > +
> > + if (!success) {
> > + if (!head) {
> > + synchronize_rcu();
> > + kvfree(ptr);
> > + } else {
> > + call_rcu(head, func);
> > + }
> > + }
> > }
> > EXPORT_SYMBOL_GPL(kvfree_call_rcu);
> >
> > @@ -188,4 +339,8 @@ void __init rcu_init(void)
> > open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
> > rcu_early_boot_tests();
> > srcu_init();
> > +
> > + INIT_DELAYED_WORK(&monitor_work, kvfree_rcu_monitor);
> > + INIT_RCU_WORK(&rcu_work, kvfree_rcu_work);
> > + static_branch_enable(&rcu_init_done);
> > }
> > --
> > 2.20.1
> >