bpf: cpumap memory prefetchw optimizations for struct page

[mirror_ubuntu-hirsute-kernel.git] / kernel / bpf / cpumap.c

/* bpf/cpumap.c
 *
 * Copyright (c) 2017 Jesper Dangaard Brouer, Red Hat Inc.
 * Released under terms in GPL version 2.  See COPYING.
 */

/* The 'cpumap' is primarily used as a backend map for XDP BPF helper
 * call bpf_redirect_map() and XDP_REDIRECT action, like 'devmap'.
 *
 * Unlike devmap which redirects XDP frames out another NIC device,
 * this map type redirects raw XDP frames to another CPU.  The remote
 * CPU will do SKB-allocation and call the normal network stack.
 *
 * This is a scalability and isolation mechanism, that allow
 * separating the early driver network XDP layer, from the rest of the
 * netstack, and assigning dedicated CPUs for this stage.  This
 * basically allows for 10G wirespeed pre-filtering via bpf.
 */
#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/ptr_ring.h>
#include <net/xdp.h>

#include <linux/sched.h>
#include <linux/workqueue.h>
#include <linux/kthread.h>
#include <linux/capability.h>
#include <trace/events/xdp.h>

#include <linux/netdevice.h>   /* netif_receive_skb_core */
#include <linux/etherdevice.h> /* eth_type_trans */

/* General idea: XDP packets getting XDP redirected to another CPU,
 * will maximum be stored/queued for one driver ->poll() call.  It is
 * guaranteed that setting flush bit and flush operation happen on
 * same CPU.  Thus, cpu_map_flush operation can deduct via this_cpu_ptr()
 * which queue in bpf_cpu_map_entry contains packets.
 */

#define CPU_MAP_BULK_SIZE 8  /* 8 == one cacheline on 64-bit archs */
struct xdp_bulk_queue {
        void *q[CPU_MAP_BULK_SIZE];
        unsigned int count;
};

/* Struct for every remote "destination" CPU in map */
struct bpf_cpu_map_entry {
        u32 cpu;    /* kthread CPU and map index */
        int map_id; /* Back reference to map */
        u32 qsize;  /* Queue size placeholder for map lookup */

        /* XDP can run multiple RX-ring queues, need __percpu enqueue store */
        struct xdp_bulk_queue __percpu *bulkq;

        /* Queue with potential multi-producers, and single-consumer kthread */
        struct ptr_ring *queue;
        struct task_struct *kthread;
        struct work_struct kthread_stop_wq;

        atomic_t refcnt; /* Control when this struct can be free'ed */
        struct rcu_head rcu;
};

struct bpf_cpu_map {
        struct bpf_map map;
        /* Below members specific for map type */
        struct bpf_cpu_map_entry **cpu_map;
        unsigned long __percpu *flush_needed;
};

static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
                             struct xdp_bulk_queue *bq, bool in_napi_ctx);

static u64 cpu_map_bitmap_size(const union bpf_attr *attr)
{
        return BITS_TO_LONGS(attr->max_entries) * sizeof(unsigned long);
}

static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
{
        struct bpf_cpu_map *cmap;
        int err = -ENOMEM;
        u64 cost;
        int ret;

        if (!capable(CAP_SYS_ADMIN))
                return ERR_PTR(-EPERM);

        /* check sanity of attributes */
        if (attr->max_entries == 0 || attr->key_size != 4 ||
            attr->value_size != 4 || attr->map_flags & ~BPF_F_NUMA_NODE)
                return ERR_PTR(-EINVAL);

        cmap = kzalloc(sizeof(*cmap), GFP_USER);
        if (!cmap)
                return ERR_PTR(-ENOMEM);

        bpf_map_init_from_attr(&cmap->map, attr);

        /* Pre-limit array size based on NR_CPUS, not final CPU check */
        if (cmap->map.max_entries > NR_CPUS) {
                err = -E2BIG;
                goto free_cmap;
        }

        /* make sure page count doesn't overflow */
        cost = (u64) cmap->map.max_entries * sizeof(struct bpf_cpu_map_entry *);
        cost += cpu_map_bitmap_size(attr) * num_possible_cpus();
        if (cost >= U32_MAX - PAGE_SIZE)
                goto free_cmap;
        cmap->map.pages = round_up(cost, PAGE_SIZE) >> PAGE_SHIFT;

        /* Notice returns -EPERM on if map size is larger than memlock limit */
        ret = bpf_map_precharge_memlock(cmap->map.pages);
        if (ret) {
                err = ret;
                goto free_cmap;
        }

        /* A per cpu bitfield with a bit per possible CPU in map  */
        cmap->flush_needed = __alloc_percpu(cpu_map_bitmap_size(attr),
                                            __alignof__(unsigned long));
        if (!cmap->flush_needed)
                goto free_cmap;

        /* Alloc array for possible remote "destination" CPUs */
        cmap->cpu_map = bpf_map_area_alloc(cmap->map.max_entries *
                                           sizeof(struct bpf_cpu_map_entry *),
                                           cmap->map.numa_node);
        if (!cmap->cpu_map)
                goto free_percpu;

        return &cmap->map;
free_percpu:
        free_percpu(cmap->flush_needed);
free_cmap:
        kfree(cmap);
        return ERR_PTR(err);
}

static void get_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
{
        atomic_inc(&rcpu->refcnt);
}

/* called from workqueue, to workaround syscall using preempt_disable */
static void cpu_map_kthread_stop(struct work_struct *work)
{
        struct bpf_cpu_map_entry *rcpu;

        rcpu = container_of(work, struct bpf_cpu_map_entry, kthread_stop_wq);

        /* Wait for flush in __cpu_map_entry_free(), via full RCU barrier,
         * as it waits until all in-flight call_rcu() callbacks complete.
         */
        rcu_barrier();

        /* kthread_stop will wake_up_process and wait for it to complete */
        kthread_stop(rcpu->kthread);
}

static struct sk_buff *cpu_map_build_skb(struct bpf_cpu_map_entry *rcpu,
                                         struct xdp_frame *xdpf,
                                         struct sk_buff *skb)
{
        unsigned int hard_start_headroom;
        unsigned int frame_size;
        void *pkt_data_start;

        /* Part of headroom was reserved to xdpf */
        hard_start_headroom = sizeof(struct xdp_frame) +  xdpf->headroom;

        /* build_skb need to place skb_shared_info after SKB end, and
         * also want to know the memory "truesize".  Thus, need to
         * know the memory frame size backing xdp_buff.
         *
         * XDP was designed to have PAGE_SIZE frames, but this
         * assumption is not longer true with ixgbe and i40e.  It
         * would be preferred to set frame_size to 2048 or 4096
         * depending on the driver.
         *   frame_size = 2048;
         *   frame_len  = frame_size - sizeof(*xdp_frame);
         *
         * Instead, with info avail, skb_shared_info in placed after
         * packet len.  This, unfortunately fakes the truesize.
         * Another disadvantage of this approach, the skb_shared_info
         * is not at a fixed memory location, with mixed length
         * packets, which is bad for cache-line hotness.
         */
        frame_size = SKB_DATA_ALIGN(xdpf->len + hard_start_headroom) +
                SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

        pkt_data_start = xdpf->data - hard_start_headroom;
        skb = build_skb_around(skb, pkt_data_start, frame_size);
        if (unlikely(!skb))
                return NULL;

        skb_reserve(skb, hard_start_headroom);
        __skb_put(skb, xdpf->len);
        if (xdpf->metasize)
                skb_metadata_set(skb, xdpf->metasize);

        /* Essential SKB info: protocol and skb->dev */
        skb->protocol = eth_type_trans(skb, xdpf->dev_rx);

        /* Optional SKB info, currently missing:
         * - HW checksum info           (skb->ip_summed)
         * - HW RX hash                 (skb_set_hash)
         * - RX ring dev queue index    (skb_record_rx_queue)
         */

        /* Allow SKB to reuse area used by xdp_frame */
        xdp_scrub_frame(xdpf);

        return skb;
}

static void __cpu_map_ring_cleanup(struct ptr_ring *ring)
{
        /* The tear-down procedure should have made sure that queue is
         * empty.  See __cpu_map_entry_replace() and work-queue
         * invoked cpu_map_kthread_stop(). Catch any broken behaviour
         * gracefully and warn once.
         */
        struct xdp_frame *xdpf;

        while ((xdpf = ptr_ring_consume(ring)))
                if (WARN_ON_ONCE(xdpf))
                        xdp_return_frame(xdpf);
}

static void put_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
{
        if (atomic_dec_and_test(&rcpu->refcnt)) {
                /* The queue should be empty at this point */
                __cpu_map_ring_cleanup(rcpu->queue);
                ptr_ring_cleanup(rcpu->queue, NULL);
                kfree(rcpu->queue);
                kfree(rcpu);
        }
}

#define CPUMAP_BATCH 8

static int cpu_map_kthread_run(void *data)
{
        struct bpf_cpu_map_entry *rcpu = data;

        set_current_state(TASK_INTERRUPTIBLE);

        /* When kthread gives stop order, then rcpu have been disconnected
         * from map, thus no new packets can enter. Remaining in-flight
         * per CPU stored packets are flushed to this queue.  Wait honoring
         * kthread_stop signal until queue is empty.
         */
        while (!kthread_should_stop() || !__ptr_ring_empty(rcpu->queue)) {
                unsigned int drops = 0, sched = 0;
                void *frames[CPUMAP_BATCH];
                void *skbs[CPUMAP_BATCH];
                gfp_t gfp = __GFP_ZERO | GFP_ATOMIC;
                int i, n, m;

                /* Release CPU reschedule checks */
                if (__ptr_ring_empty(rcpu->queue)) {
                        set_current_state(TASK_INTERRUPTIBLE);
                        /* Recheck to avoid lost wake-up */
                        if (__ptr_ring_empty(rcpu->queue)) {
                                schedule();
                                sched = 1;
                        } else {
                                __set_current_state(TASK_RUNNING);
                        }
                } else {
                        sched = cond_resched();
                }

                /*
                 * The bpf_cpu_map_entry is single consumer, with this
                 * kthread CPU pinned. Lockless access to ptr_ring
                 * consume side valid as no-resize allowed of queue.
                 */
                n = ptr_ring_consume_batched(rcpu->queue, frames, CPUMAP_BATCH);

                for (i = 0; i < n; i++) {
                        void *f = frames[i];
                        struct page *page = virt_to_page(f);

                        /* Bring struct page memory area to curr CPU. Read by
                         * build_skb_around via page_is_pfmemalloc(), and when
                         * freed written by page_frag_free call.
                         */
                        prefetchw(page);
                }

                m = kmem_cache_alloc_bulk(skbuff_head_cache, gfp, n, skbs);
                if (unlikely(m == 0)) {
                        for (i = 0; i < n; i++)
                                skbs[i] = NULL; /* effect: xdp_return_frame */
                        drops = n;
                }

                local_bh_disable();
                for (i = 0; i < n; i++) {
                        struct xdp_frame *xdpf = frames[i];
                        struct sk_buff *skb = skbs[i];
                        int ret;

                        skb = cpu_map_build_skb(rcpu, xdpf, skb);
                        if (!skb) {
                                xdp_return_frame(xdpf);
                                continue;
                        }

                        /* Inject into network stack */
                        ret = netif_receive_skb_core(skb);
                        if (ret == NET_RX_DROP)
                                drops++;
                }
                /* Feedback loop via tracepoint */
                trace_xdp_cpumap_kthread(rcpu->map_id, n, drops, sched);

                local_bh_enable(); /* resched point, may call do_softirq() */
        }
        __set_current_state(TASK_RUNNING);

        put_cpu_map_entry(rcpu);
        return 0;
}

static struct bpf_cpu_map_entry *__cpu_map_entry_alloc(u32 qsize, u32 cpu,
                                                       int map_id)
{
        gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
        struct bpf_cpu_map_entry *rcpu;
        int numa, err;

        /* Have map->numa_node, but choose node of redirect target CPU */
        numa = cpu_to_node(cpu);

        rcpu = kzalloc_node(sizeof(*rcpu), gfp, numa);
        if (!rcpu)
                return NULL;

        /* Alloc percpu bulkq */
        rcpu->bulkq = __alloc_percpu_gfp(sizeof(*rcpu->bulkq),
                                         sizeof(void *), gfp);
        if (!rcpu->bulkq)
                goto free_rcu;

        /* Alloc queue */
        rcpu->queue = kzalloc_node(sizeof(*rcpu->queue), gfp, numa);
        if (!rcpu->queue)
                goto free_bulkq;

        err = ptr_ring_init(rcpu->queue, qsize, gfp);
        if (err)
                goto free_queue;

        rcpu->cpu    = cpu;
        rcpu->map_id = map_id;
        rcpu->qsize  = qsize;

        /* Setup kthread */
        rcpu->kthread = kthread_create_on_node(cpu_map_kthread_run, rcpu, numa,
                                               "cpumap/%d/map:%d", cpu, map_id);
        if (IS_ERR(rcpu->kthread))
                goto free_ptr_ring;

        get_cpu_map_entry(rcpu); /* 1-refcnt for being in cmap->cpu_map[] */
        get_cpu_map_entry(rcpu); /* 1-refcnt for kthread */

        /* Make sure kthread runs on a single CPU */
        kthread_bind(rcpu->kthread, cpu);
        wake_up_process(rcpu->kthread);

        return rcpu;

free_ptr_ring:
        ptr_ring_cleanup(rcpu->queue, NULL);
free_queue:
        kfree(rcpu->queue);
free_bulkq:
        free_percpu(rcpu->bulkq);
free_rcu:
        kfree(rcpu);
        return NULL;
}

static void __cpu_map_entry_free(struct rcu_head *rcu)
{
        struct bpf_cpu_map_entry *rcpu;
        int cpu;

        /* This cpu_map_entry have been disconnected from map and one
         * RCU graze-period have elapsed.  Thus, XDP cannot queue any
         * new packets and cannot change/set flush_needed that can
         * find this entry.
         */
        rcpu = container_of(rcu, struct bpf_cpu_map_entry, rcu);

        /* Flush remaining packets in percpu bulkq */
        for_each_online_cpu(cpu) {
                struct xdp_bulk_queue *bq = per_cpu_ptr(rcpu->bulkq, cpu);

                /* No concurrent bq_enqueue can run at this point */
                bq_flush_to_queue(rcpu, bq, false);
        }
        free_percpu(rcpu->bulkq);
        /* Cannot kthread_stop() here, last put free rcpu resources */
        put_cpu_map_entry(rcpu);
}

/* After xchg pointer to bpf_cpu_map_entry, use the call_rcu() to
 * ensure any driver rcu critical sections have completed, but this
 * does not guarantee a flush has happened yet. Because driver side
 * rcu_read_lock/unlock only protects the running XDP program.  The
 * atomic xchg and NULL-ptr check in __cpu_map_flush() makes sure a
 * pending flush op doesn't fail.
 *
 * The bpf_cpu_map_entry is still used by the kthread, and there can
 * still be pending packets (in queue and percpu bulkq).  A refcnt
 * makes sure to last user (kthread_stop vs. call_rcu) free memory
 * resources.
 *
 * The rcu callback __cpu_map_entry_free flush remaining packets in
 * percpu bulkq to queue.  Due to caller map_delete_elem() disable
 * preemption, cannot call kthread_stop() to make sure queue is empty.
 * Instead a work_queue is started for stopping kthread,
 * cpu_map_kthread_stop, which waits for an RCU graze period before
 * stopping kthread, emptying the queue.
 */
static void __cpu_map_entry_replace(struct bpf_cpu_map *cmap,
                                    u32 key_cpu, struct bpf_cpu_map_entry *rcpu)
{
        struct bpf_cpu_map_entry *old_rcpu;

        old_rcpu = xchg(&cmap->cpu_map[key_cpu], rcpu);
        if (old_rcpu) {
                call_rcu(&old_rcpu->rcu, __cpu_map_entry_free);
                INIT_WORK(&old_rcpu->kthread_stop_wq, cpu_map_kthread_stop);
                schedule_work(&old_rcpu->kthread_stop_wq);
        }
}

static int cpu_map_delete_elem(struct bpf_map *map, void *key)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        u32 key_cpu = *(u32 *)key;

        if (key_cpu >= map->max_entries)
                return -EINVAL;

        /* notice caller map_delete_elem() use preempt_disable() */
        __cpu_map_entry_replace(cmap, key_cpu, NULL);
        return 0;
}

static int cpu_map_update_elem(struct bpf_map *map, void *key, void *value,
                               u64 map_flags)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        struct bpf_cpu_map_entry *rcpu;

        /* Array index key correspond to CPU number */
        u32 key_cpu = *(u32 *)key;
        /* Value is the queue size */
        u32 qsize = *(u32 *)value;

        if (unlikely(map_flags > BPF_EXIST))
                return -EINVAL;
        if (unlikely(key_cpu >= cmap->map.max_entries))
                return -E2BIG;
        if (unlikely(map_flags == BPF_NOEXIST))
                return -EEXIST;
        if (unlikely(qsize > 16384)) /* sanity limit on qsize */
                return -EOVERFLOW;

        /* Make sure CPU is a valid possible cpu */
        if (!cpu_possible(key_cpu))
                return -ENODEV;

        if (qsize == 0) {
                rcpu = NULL; /* Same as deleting */
        } else {
                /* Updating qsize cause re-allocation of bpf_cpu_map_entry */
                rcpu = __cpu_map_entry_alloc(qsize, key_cpu, map->id);
                if (!rcpu)
                        return -ENOMEM;
        }
        rcu_read_lock();
        __cpu_map_entry_replace(cmap, key_cpu, rcpu);
        rcu_read_unlock();
        return 0;
}

static void cpu_map_free(struct bpf_map *map)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        int cpu;
        u32 i;

        /* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
         * so the bpf programs (can be more than one that used this map) were
         * disconnected from events. Wait for outstanding critical sections in
         * these programs to complete. The rcu critical section only guarantees
         * no further "XDP/bpf-side" reads against bpf_cpu_map->cpu_map.
         * It does __not__ ensure pending flush operations (if any) are
         * complete.
         */

        bpf_clear_redirect_map(map);
        synchronize_rcu();

        /* To ensure all pending flush operations have completed wait for flush
         * bitmap to indicate all flush_needed bits to be zero on _all_ cpus.
         * Because the above synchronize_rcu() ensures the map is disconnected
         * from the program we can assume no new bits will be set.
         */
        for_each_online_cpu(cpu) {
                unsigned long *bitmap = per_cpu_ptr(cmap->flush_needed, cpu);

                while (!bitmap_empty(bitmap, cmap->map.max_entries))
                        cond_resched();
        }

        /* For cpu_map the remote CPUs can still be using the entries
         * (struct bpf_cpu_map_entry).
         */
        for (i = 0; i < cmap->map.max_entries; i++) {
                struct bpf_cpu_map_entry *rcpu;

                rcpu = READ_ONCE(cmap->cpu_map[i]);
                if (!rcpu)
                        continue;

                /* bq flush and cleanup happens after RCU graze-period */
                __cpu_map_entry_replace(cmap, i, NULL); /* call_rcu */
        }
        free_percpu(cmap->flush_needed);
        bpf_map_area_free(cmap->cpu_map);
        kfree(cmap);
}

struct bpf_cpu_map_entry *__cpu_map_lookup_elem(struct bpf_map *map, u32 key)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        struct bpf_cpu_map_entry *rcpu;

        if (key >= map->max_entries)
                return NULL;

        rcpu = READ_ONCE(cmap->cpu_map[key]);
        return rcpu;
}

static void *cpu_map_lookup_elem(struct bpf_map *map, void *key)
{
        struct bpf_cpu_map_entry *rcpu =
                __cpu_map_lookup_elem(map, *(u32 *)key);

        return rcpu ? &rcpu->qsize : NULL;
}

static int cpu_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        u32 index = key ? *(u32 *)key : U32_MAX;
        u32 *next = next_key;

        if (index >= cmap->map.max_entries) {
                *next = 0;
                return 0;
        }

        if (index == cmap->map.max_entries - 1)
                return -ENOENT;
        *next = index + 1;
        return 0;
}

const struct bpf_map_ops cpu_map_ops = {
        .map_alloc              = cpu_map_alloc,
        .map_free               = cpu_map_free,
        .map_delete_elem        = cpu_map_delete_elem,
        .map_update_elem        = cpu_map_update_elem,
        .map_lookup_elem        = cpu_map_lookup_elem,
        .map_get_next_key       = cpu_map_get_next_key,
        .map_check_btf          = map_check_no_btf,
};

static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
                             struct xdp_bulk_queue *bq, bool in_napi_ctx)
{
        unsigned int processed = 0, drops = 0;
        const int to_cpu = rcpu->cpu;
        struct ptr_ring *q;
        int i;

        if (unlikely(!bq->count))
                return 0;

        q = rcpu->queue;
        spin_lock(&q->producer_lock);

        for (i = 0; i < bq->count; i++) {
                struct xdp_frame *xdpf = bq->q[i];
                int err;

                err = __ptr_ring_produce(q, xdpf);
                if (err) {
                        drops++;
                        if (likely(in_napi_ctx))
                                xdp_return_frame_rx_napi(xdpf);
                        else
                                xdp_return_frame(xdpf);
                }
                processed++;
        }
        bq->count = 0;
        spin_unlock(&q->producer_lock);

        /* Feedback loop via tracepoints */
        trace_xdp_cpumap_enqueue(rcpu->map_id, processed, drops, to_cpu);
        return 0;
}

/* Runs under RCU-read-side, plus in softirq under NAPI protection.
 * Thus, safe percpu variable access.
 */
static int bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf)
{
        struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);

        if (unlikely(bq->count == CPU_MAP_BULK_SIZE))
                bq_flush_to_queue(rcpu, bq, true);

        /* Notice, xdp_buff/page MUST be queued here, long enough for
         * driver to code invoking us to finished, due to driver
         * (e.g. ixgbe) recycle tricks based on page-refcnt.
         *
         * Thus, incoming xdp_frame is always queued here (else we race
         * with another CPU on page-refcnt and remaining driver code).
         * Queue time is very short, as driver will invoke flush
         * operation, when completing napi->poll call.
         */
        bq->q[bq->count++] = xdpf;
        return 0;
}

int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp,
                    struct net_device *dev_rx)
{
        struct xdp_frame *xdpf;

        xdpf = convert_to_xdp_frame(xdp);
        if (unlikely(!xdpf))
                return -EOVERFLOW;

        /* Info needed when constructing SKB on remote CPU */
        xdpf->dev_rx = dev_rx;

        bq_enqueue(rcpu, xdpf);
        return 0;
}

void __cpu_map_insert_ctx(struct bpf_map *map, u32 bit)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        unsigned long *bitmap = this_cpu_ptr(cmap->flush_needed);

        __set_bit(bit, bitmap);
}

void __cpu_map_flush(struct bpf_map *map)
{
        struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
        unsigned long *bitmap = this_cpu_ptr(cmap->flush_needed);
        u32 bit;

        /* The napi->poll softirq makes sure __cpu_map_insert_ctx()
         * and __cpu_map_flush() happen on same CPU. Thus, the percpu
         * bitmap indicate which percpu bulkq have packets.
         */
        for_each_set_bit(bit, bitmap, map->max_entries) {
                struct bpf_cpu_map_entry *rcpu = READ_ONCE(cmap->cpu_map[bit]);
                struct xdp_bulk_queue *bq;

                /* This is possible if entry is removed by user space
                 * between xdp redirect and flush op.
                 */
                if (unlikely(!rcpu))
                        continue;

                __clear_bit(bit, bitmap);

                /* Flush all frames in bulkq to real queue */
                bq = this_cpu_ptr(rcpu->bulkq);
                bq_flush_to_queue(rcpu, bq, true);

                /* If already running, costs spin_lock_irqsave + smb_mb */
                wake_up_process(rcpu->kthread);
        }
}