Re: [PATCH 1/1] rcu/tree: support kfree_bulk() interface in kfree_rcu()

From: Paul E. McKenney
Date: Wed Jan 15 2020 - 21:41:31 EST

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In reply to: Joel Fernandes: "Re: [PATCH 1/1] rcu/tree: support kfree_bulk() interface in kfree_rcu()"
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On Wed, Jan 15, 2020 at 08:14:10PM -0500, Joel Fernandes wrote:
> On Tue, Dec 31, 2019 at 01:22:41PM +0100, Uladzislau Rezki (Sony) wrote:
> > kfree_rcu() logic can be improved further by using kfree_bulk()
> > interface along with "basic batching support" introduced earlier.
> >
> > The are at least two advantages of using "bulk" interface:
> > - in case of large number of kfree_rcu() requests kfree_bulk()
> > reduces the per-object overhead caused by calling kfree()
> > per-object.
> >
> > - reduces the number of cache-misses due to "pointer chasing"
> > between objects which can be far spread between each other.
> >
> > This approach defines a new kfree_rcu_bulk_data structure that
> > stores pointers in an array with a specific size. Number of entries
> > in that array depends on PAGE_SIZE making kfree_rcu_bulk_data
> > structure to be exactly one page.
> >
> > Since it deals with "block-chain" technique there is an extra
> > need in dynamic allocation when a new block is required. Memory
> > is allocated with GFP_NOWAIT | __GFP_NOWARN flags, i.e. that
> > allows to skip direct reclaim under low memory condition to
> > prevent stalling and fails silently under high memory pressure.
> >
> > The "emergency path" gets maintained when a system is run out
> > of memory. In that case objects are linked into regular list
> > and that is it.
> >
> > In order to evaluate it, the "rcuperf" was run to analyze how
> > much memory is consumed and what is kfree_bulk() throughput.
> >
> > Testing on the HiKey-960, arm64, 8xCPUs with below parameters:
> >
> > CONFIG_SLAB=y
> > kfree_loops=200000 kfree_alloc_num=1000 kfree_rcu_test=1
> >
> > 102898760401 ns, loops: 200000, batches: 5822, memory footprint: 158MB
> > 89947009882 ns, loops: 200000, batches: 6715, memory footprint: 115MB
> >
> > rcuperf shows approximately ~12% better throughput(Total time)
> > in case of using "bulk" interface. The "drain logic" or its RCU
> > callback does the work faster that leads to better throughput.
>
> Tested-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> (Vlad is going to post a v2 which fixes a debugobjects bug but that should
> not have any impact on testing).

Very good! Uladzislau, could you please add Joel's Tested-by in
your next posting?

Thanx, Paul

> thanks,
>
> - Joel
>
>
>
> >
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > ---
> > kernel/rcu/tree.c | 154 ++++++++++++++++++++++++++++++++++++++--------
> > 1 file changed, 130 insertions(+), 24 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 48fba2257748..4ee5c737558b 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2754,22 +2754,45 @@ EXPORT_SYMBOL_GPL(call_rcu);
> > #define KFREE_DRAIN_JIFFIES (HZ / 50)
> > #define KFREE_N_BATCHES 2
> >
> > +/*
> > + * This macro defines how many entries the "records" array
> > + * will contain. It is based on the fact that the size of
> > + * kfree_rcu_bulk_data structure becomes exactly one page.
> > + */
> > +#define KFREE_BULK_MAX_ENTR ((PAGE_SIZE / sizeof(void *)) - 2)
> > +
> > +/**
> > + * struct kfree_rcu_bulk_data - single block to store kfree_rcu() pointers
> > + * @nr_records: Number of active pointers in the array
> > + * @records: Array of the kfree_rcu() pointers
> > + * @next: Next bulk object in the block chain
> > + */
> > +struct kfree_rcu_bulk_data {
> > + unsigned long nr_records;
> > + void *records[KFREE_BULK_MAX_ENTR];
> > + struct kfree_rcu_bulk_data *next;
> > +};
> > +
> > /**
> > * struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests
> > * @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period
> > * @head_free: List of kfree_rcu() objects waiting for a grace period
> > + * @bhead_free: Bulk-List of kfree_rcu() objects waiting for a grace period
> > * @krcp: Pointer to @kfree_rcu_cpu structure
> > */
> >
> > struct kfree_rcu_cpu_work {
> > struct rcu_work rcu_work;
> > struct rcu_head *head_free;
> > + struct kfree_rcu_bulk_data *bhead_free;
> > struct kfree_rcu_cpu *krcp;
> > };
> >
> > /**
> > * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
> > * @head: List of kfree_rcu() objects not yet waiting for a grace period
> > + * @bhead: Bulk-List of kfree_rcu() objects not yet waiting for a grace period
> > + * @bcached: Keeps at most one object for later reuse when build chain blocks
> > * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
> > * @lock: Synchronize access to this structure
> > * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> > @@ -2783,6 +2806,8 @@ struct kfree_rcu_cpu_work {
> > */
> > struct kfree_rcu_cpu {
> > struct rcu_head *head;
> > + struct kfree_rcu_bulk_data *bhead;
> > + struct kfree_rcu_bulk_data *bcached;
> > struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> > spinlock_t lock;
> > struct delayed_work monitor_work;
> > @@ -2800,6 +2825,7 @@ static void kfree_rcu_work(struct work_struct *work)
> > {
> > unsigned long flags;
> > struct rcu_head *head, *next;
> > + struct kfree_rcu_bulk_data *bhead, *bnext;
> > struct kfree_rcu_cpu *krcp;
> > struct kfree_rcu_cpu_work *krwp;
> >
> > @@ -2809,22 +2835,39 @@ static void kfree_rcu_work(struct work_struct *work)
> > spin_lock_irqsave(&krcp->lock, flags);
> > head = krwp->head_free;
> > krwp->head_free = NULL;
> > + bhead = krwp->bhead_free;
> > + krwp->bhead_free = NULL;
> > spin_unlock_irqrestore(&krcp->lock, flags);
> >
> > - // List "head" is now private, so traverse locklessly.
> > + /* List "bhead" is now private, so traverse locklessly. */
> > + for (; bhead; bhead = bnext) {
> > + bnext = bhead->next;
> > +
> > + rcu_lock_acquire(&rcu_callback_map);
> > + kfree_bulk(bhead->nr_records, bhead->records);
> > + rcu_lock_release(&rcu_callback_map);
> > +
> > + if (cmpxchg(&krcp->bcached, NULL, bhead))
> > + free_page((unsigned long) bhead);
> > +
> > + cond_resched_tasks_rcu_qs();
> > + }
> > +
> > + /*
> > + * Emergency case only. It can happen under low memory
> > + * condition when an allocation gets failed, so the "bulk"
> > + * path can not be temporary maintained.
> > + */
> > for (; head; head = next) {
> > unsigned long offset = (unsigned long)head->func;
> >
> > next = head->next;
> > - // Potentially optimize with kfree_bulk in future.
> > debug_rcu_head_unqueue(head);
> > rcu_lock_acquire(&rcu_callback_map);
> > trace_rcu_invoke_kfree_callback(rcu_state.name, head, offset);
> >
> > - if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset))) {
> > - /* Could be optimized with kfree_bulk() in future. */
> > + if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset)))
> > kfree((void *)head - offset);
> > - }
> >
> > rcu_lock_release(&rcu_callback_map);
> > cond_resched_tasks_rcu_qs();
> > @@ -2839,26 +2882,45 @@ static void kfree_rcu_work(struct work_struct *work)
> > */
> > static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
> > {
> > + struct kfree_rcu_cpu_work *krwp;
> > + bool queued = false;
> > int i;
> > - struct kfree_rcu_cpu_work *krwp = NULL;
> >
> > lockdep_assert_held(&krcp->lock);
> > - for (i = 0; i < KFREE_N_BATCHES; i++)
> > - if (!krcp->krw_arr[i].head_free) {
> > - krwp = &(krcp->krw_arr[i]);
> > - break;
> > - }
> >
> > - // If a previous RCU batch is in progress, we cannot immediately
> > - // queue another one, so return false to tell caller to retry.
> > - if (!krwp)
> > - return false;
> > + for (i = 0; i < KFREE_N_BATCHES; i++) {
> > + krwp = &(krcp->krw_arr[i]);
> >
> > - krwp->head_free = krcp->head;
> > - krcp->head = NULL;
> > - INIT_RCU_WORK(&krwp->rcu_work, kfree_rcu_work);
> > - queue_rcu_work(system_wq, &krwp->rcu_work);
> > - return true;
> > + /*
> > + * Try to detach bhead or head and attach it over any
> > + * available corresponding free channel. It can be that
> > + * a previous RCU batch is in progress, it means that
> > + * immediately to queue another one is not possible so
> > + * return false to tell caller to retry.
> > + */
> > + if ((krcp->bhead && !krwp->bhead_free) ||
> > + (krcp->head && !krwp->head_free)) {
> > + if (!krwp->bhead_free) {
> > + krwp->bhead_free = krcp->bhead;
> > + krcp->bhead = NULL;
> > + }
> > +
> > + if (!krwp->head_free) {
> > + krwp->head_free = krcp->head;
> > + krcp->head = NULL;
> > + }
> > +
> > + /*
> > + * The work can already be queued. If so, it means that
> > + * within a short time, second, either head or bhead has
> > + * been detached as well.
> > + */
> > + queue_rcu_work(system_wq, &krwp->rcu_work);
> > + queued = true;
> > + }
> > + }
> > +
> > + return queued;
> > }
> >
> > static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
> > @@ -2895,6 +2957,39 @@ static void kfree_rcu_monitor(struct work_struct *work)
> > spin_unlock_irqrestore(&krcp->lock, flags);
> > }
> >
> > +static inline bool
> > +kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > +{
> > + struct kfree_rcu_bulk_data *bnode;
> > +
> > + if (unlikely(!krcp->initialized))
> > + return false;
> > +
> > + lockdep_assert_held(&krcp->lock);
> > +
> > + /* Check if a new block is required. */
> > + if (!krcp->bhead ||
> > + krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
> > + bnode = xchg(&krcp->bcached, NULL);
> > + if (!bnode)
> > + bnode = (struct kfree_rcu_bulk_data *)
> > + __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > +
> > + /* No cache or an allocation got failed. */
> > + if (unlikely(!bnode))
> > + return false;
> > +
> > + /* Initialize the new block. */
> > + bnode->nr_records = 0;
> > + bnode->next = krcp->bhead;
> > + krcp->bhead = bnode;
> > + }
> > +
> > + /* Finally insert. */
> > + krcp->bhead->records[krcp->bhead->nr_records++] = ptr;
> > + return true;
> > +}
> > +
> > /*
> > * Queue a request for lazy invocation of kfree() after a grace period.
> > *
> > @@ -2926,9 +3021,17 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > __func__, head);
> > goto unlock_return;
> > }
> > - head->func = func;
> > - head->next = krcp->head;
> > - krcp->head = head;
> > +
> > + /*
> > + * Under high memory pressure GFP_NOWAIT can fail,
> > + * in that case the emergency path is maintained.
> > + */
> > + if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp,
> > + (void *) head - (unsigned long) func))) {
> > + head->func = func;
> > + head->next = krcp->head;
> > + krcp->head = head;
> > + }
> >
> > // Set timer to drain after KFREE_DRAIN_JIFFIES.
> > if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
> > @@ -3834,8 +3937,11 @@ static void __init kfree_rcu_batch_init(void)
> > struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> >
> > spin_lock_init(&krcp->lock);
> > - for (i = 0; i < KFREE_N_BATCHES; i++)
> > + for (i = 0; i < KFREE_N_BATCHES; i++) {
> > + INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
> > krcp->krw_arr[i].krcp = krcp;
> > + }
> > +
> > INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> > krcp->initialized = true;
> > }
> > --
> > 2.20.1
> >

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In reply to: Joel Fernandes: "Re: [PATCH 1/1] rcu/tree: support kfree_bulk() interface in kfree_rcu()"
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