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

Paul E. McKenney
2019-08-14T23:02:33Z

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@joelfernandes.org>
> Subject: [PATCH] WIP: Multiple batches
>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> 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
>