Re: [PATCH v3 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Paul E. McKenney]

On Wed, Aug 14, 2019 at 06:34:13PM -0400, Joel Fernandes wrote:
> On Wed, Aug 14, 2019 at 11:44:29AM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 14, 2019 at 01:22:33PM -0400, Joel Fernandes wrote:
> > > On Wed, Aug 14, 2019 at 10:38:17AM -0400, Joel Fernandes wrote:
> > > > On Tue, Aug 13, 2019 at 12:07:38PM -0700, Paul E. McKenney wrote:
> > >  [snip]
> > > > > > - * Queue an RCU callback for lazy invocation after a grace period.
> > > > > > - * This will likely be later named something like "call_rcu_lazy()",
> > > > > > - * but this change will require some way of tagging the lazy RCU
> > > > > > - * callbacks in the list of pending callbacks. Until then, this
> > > > > > - * function may only be called from __kfree_rcu().
> > > > > > + * Maximum number of kfree(s) to batch, if this limit is hit then the batch of
> > > > > > + * kfree(s) is queued for freeing after a grace period, right away.
> > > > > >   */
> > > > > > -void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > > > > +struct kfree_rcu_cpu {
> > > > > > +	/* The rcu_work node for queuing work with queue_rcu_work(). The work
> > > > > > +	 * is done after a grace period.
> > > > > > +	 */
> > > > > > +	struct rcu_work rcu_work;
> > > > > > +
> > > > > > +	/* The list of objects being queued in a batch but are not yet
> > > > > > +	 * scheduled to be freed.
> > > > > > +	 */
> > > > > > +	struct rcu_head *head;
> > > > > > +
> > > > > > +	/* The list of objects that have now left ->head and are queued for
> > > > > > +	 * freeing after a grace period.
> > > > > > +	 */
> > > > > > +	struct rcu_head *head_free;
> > > > > 
> > > > > So this is not yet the one that does multiple batches concurrently
> > > > > awaiting grace periods, correct?  Or am I missing something subtle?
> > > > 
> > > > Yes, it is not. I honestly, still did not understand that idea. Or how it
> > > > would improve things. May be we can discuss at LPC on pen and paper? But I
> > > > think that can also be a follow-up optimization.
> > > 
> > > I got it now. Basically we can benefit a bit more by having another list
> > > (that is have multiple kfree_rcu batches in flight). I will think more about
> > > it - but hopefully we don't need to gate this patch by that.
> > 
> > I am willing to take this as a later optimization.
> > 
> > > It'll be interesting to see what rcuperf says about such an improvement :)
> > 
> > Indeed, no guarantees either way.  The reason for hope assumes a busy
> > system where each grace period is immediately followed by another
> > grace period.  On such a system, the current setup allows each CPU to
> > make use only of every second grace period for its kfree_rcu() work.
> > The hope would therefore be that this would reduce the memory footprint
> > substantially with no increase in overhead.
> 
> Good news! I was able to bring down memory foot print by almost 30% by adding
> another batch. Below is the patch. Thanks for the suggestion!

Nice!

> I can add this as a patch on top of the initial one, for easier review.

Yes, please!

> The memory consumed drops from 300-350MB to 200-250MB. Increasing
> KFREE_N_BATCHES did not cause a reduction in memory, though.

OK, good to know.

						Thanx, Paul

> ---8<-----------------------
> 
> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
> Subject: [PATCH] WIP: Multiple batches
> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 58 +++++++++++++++++++++++++++++++++--------------
>  1 file changed, 41 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 1d1847cadea2..a272c893dbdc 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2596,26 +2596,35 @@ EXPORT_SYMBOL_GPL(call_rcu);
>  
>  /* Maximum number of jiffies to wait before draining a batch. */
>  #define KFREE_DRAIN_JIFFIES (HZ / 50)
> +#define KFREE_N_BATCHES 2
> +
> +struct kfree_rcu_work {
> +	/* The rcu_work node for queuing work with queue_rcu_work(). The work
> +	 * is done after a grace period.
> +	 */
> +	struct rcu_work rcu_work;
> +
> +	/* The list of objects that have now left ->head and are queued for
> +	 * freeing after a grace period.
> +	 */
> +	struct rcu_head *head_free;
> +
> +	struct kfree_rcu_cpu *krc;
> +};
> +static DEFINE_PER_CPU(__typeof__(struct kfree_rcu_work)[KFREE_N_BATCHES], krw);
>  
>  /*
>   * Maximum number of kfree(s) to batch, if this limit is hit then the batch of
>   * kfree(s) is queued for freeing after a grace period, right away.
>   */
>  struct kfree_rcu_cpu {
> -	/* The rcu_work node for queuing work with queue_rcu_work(). The work
> -	 * is done after a grace period.
> -	 */
> -	struct rcu_work rcu_work;
>  
>  	/* The list of objects being queued in a batch but are not yet
>  	 * scheduled to be freed.
>  	 */
>  	struct rcu_head *head;
>  
> -	/* The list of objects that have now left ->head and are queued for
> -	 * freeing after a grace period.
> -	 */
> -	struct rcu_head *head_free;
> +	struct kfree_rcu_work *krw;
>  
>  	/* Protect concurrent access to this structure. */
>  	spinlock_t lock;
> @@ -2638,12 +2647,15 @@ 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 *krw = container_of(to_rcu_work(work),
> +					struct kfree_rcu_work, rcu_work);
> +	struct kfree_rcu_cpu *krcp;
> +
> +	krcp = krw->krc;
>  
>  	spin_lock_irqsave(&krcp->lock, flags);
> -	head = krcp->head_free;
> -	krcp->head_free = NULL;
> +	head = krw->head_free;
> +	krw->head_free = NULL;
>  	spin_unlock_irqrestore(&krcp->lock, flags);
>  
>  	/*
> @@ -2666,19 +2678,30 @@ 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 *krw = NULL;
> +
>  	lockdep_assert_held(&krcp->lock);
> +	while (i < KFREE_N_BATCHES) {
> +		if (!krcp->krw[i].head_free) {
> +			krw = &(krcp->krw[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 (krcp->head_free)
> +	if (!krw)
>  		return false;
>  
> -	krcp->head_free = krcp->head;
> +	krw->head_free = krcp->head;
> +	krw->krc = krcp;   /* Should need to do only once, optimize later. */
>  	krcp->head = NULL;
> -	INIT_RCU_WORK(&krcp->rcu_work, kfree_rcu_work);
> -	queue_rcu_work(system_wq, &krcp->rcu_work);
> +	INIT_RCU_WORK(&krw->rcu_work, kfree_rcu_work);
> +	queue_rcu_work(system_wq, &krw->rcu_work);
>  
>  	return true;
>  }
> @@ -3631,6 +3654,7 @@ static void __init kfree_rcu_batch_init(void)
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>  
>  		spin_lock_init(&krcp->lock);
> +		krcp->krw = &(per_cpu(krw, cpu)[0]);
>  		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
>  	}
>  }
> -- 
> 2.23.0.rc1.153.gdeed80330f-goog
>