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 >