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Date:   Sun, 3 May 2020 20:27:00 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 22/24] rcu/tiny: support reclaim for head-less object

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@...lfernandes.org>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.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
> > 

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