Re: [PATCH v2] rcu/tree: Add multiple in-flight batches of kfree_rcu work

[Joel Fernandes]

On Wed, Aug 28, 2019 at 01:45:21PM -0700, Paul E. McKenney wrote:
> On Wed, Aug 28, 2019 at 10:09:52AM -0400, Joel Fernandes (Google) wrote:
> > During testing, it was observed that amount of memory consumed due
> > kfree_rcu() batching is 300-400MB. Previously we had only a single
> > head_free pointer pointing to the list of rcu_head(s) that are to be
> > freed after a grace period. Until this list is drained, we cannot queue
> > any more objects on it since such objects may not be ready to be
> > reclaimed when the worker thread eventually gets to drainin g the
> > head_free list.
> > 
> > We can do better by maintaining multiple lists as done by this patch.
> > Testing shows that memory consumption came down by around 100-150MB with
> > just adding another list. Adding more than 1 additional list did not
> > show any improvement.
[snip]
> > @@ -2730,12 +2739,14 @@ static void kfree_rcu_work(struct work_struct *work)
> >  {
> >  	unsigned long flags;
> >  	struct rcu_head *head, *next;
> > -	struct kfree_rcu_cpu *krcp = container_of(to_rcu_work(work),
> > -					struct kfree_rcu_cpu, rcu_work);
> > +	struct kfree_rcu_work *krwp = container_of(to_rcu_work(work),
> > +					struct kfree_rcu_work, rcu_work);
> > +	struct kfree_rcu_cpu *krcp;
> > +
> > +	krcp = krwp->krcp;
> >  
> >  	spin_lock_irqsave(&krcp->lock, flags);
> > -	head = krcp->head_free;
> > -	krcp->head_free = NULL;
> > +	head = xchg(&krwp->head_free, NULL);
> 
> Given that we hold the lock, why the xchg()?  Alternatively, why not
> just acquire the lock in the other places you use xchg()?  This is a
> per-CPU lock, so contention should not be a problem, should it?

I realized I was being silly :(. Was trying to reduce lines of code and hence
implemented it like that as a one-liner. Locking protocol is not needed or
intended for that xchg since as pointed, a lock is held.

> >  	spin_unlock_irqrestore(&krcp->lock, flags);
> >  
> >  	/*
> > @@ -2758,19 +2769,28 @@ static void kfree_rcu_work(struct work_struct *work)
> >   */
> >  static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
> >  {
> > +	int i = 0;
> > +	struct kfree_rcu_work *krwp = NULL;
> > +
> >  	lockdep_assert_held(&krcp->lock);
> > +	while (i < KFREE_N_BATCHES) {
> > +		if (!krcp->krw_arr[i].head_free) {
> > +			krwp = &(krcp->krw_arr[i]);
> > +			break;
> > +		}
> > +		i++;
> > +	}
> >  
> > -	/* If a previous RCU batch work is already in progress, we cannot queue
> > +	/* If both RCU batches are already in progress, we cannot queue
> >  	 * another one, just refuse the optimization and it will be retried
> >  	 * again in KFREE_DRAIN_JIFFIES time.
> >  	 */
> 
> If you are going to remove the traditional first "/*" line of a comment,
> why not go all the way and cut the last one as well?  "//".

Will add the /* in the beginning :)

> > -	if (krcp->head_free)
> > +	if (!krwp)
> >  		return false;
> >  
> > -	krcp->head_free = krcp->head;
> > -	krcp->head = NULL;
> > -	INIT_RCU_WORK(&krcp->rcu_work, kfree_rcu_work);
> > -	queue_rcu_work(system_wq, &krcp->rcu_work);
> > +	krwp->head_free = xchg(&krcp->head, NULL);
> 
> This isn't anywhere near a fastpath, so just acquiring the lock is a
> better choice here.

My reasoning was same as above. Will change it to 2 statements since lock is
already held.

> > +	INIT_RCU_WORK(&krwp->rcu_work, kfree_rcu_work);
> > +	queue_rcu_work(system_wq, &krwp->rcu_work);
> >  
> >  	return true;
> >  }
> > @@ -3736,8 +3756,11 @@ static void __init kfree_rcu_batch_init(void)
> >  
> >  	for_each_possible_cpu(cpu) {
> >  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > +		int i = KFREE_N_BATCHES;
> >  
> >  		spin_lock_init(&krcp->lock);
> > +		while (i--)
> > +			krcp->krw_arr[i].krcp = krcp;
> 
> This was indeed a nice trick back in the PDP-11 days of 64-kilobyte
> address spaces, so thank you for the nostalgia!  However, a straight-up
> "for" loop is less vulnerable to code being added between the declaration
> of "i" and the "while" loop.

Ok, will do.

thanks,

 - Joel