linux/kernel/bpf/cpumap.c
Jesper Dangaard Brouer 86d231459d bpf: cpumap memory prefetchw optimizations for struct page
A lot of the performance gain comes from this patch.

While analysing performance overhead it was found that the largest CPU
stalls were caused when touching the struct page area. It is first read with
a READ_ONCE from build_skb_around via page_is_pfmemalloc(), and when freed
written by page_frag_free() call.

Measurements show that the prefetchw (W) variant operation is needed to
achieve the performance gain. We believe this optimization it two fold,
first the W-variant saves one step in the cache-coherency protocol, and
second it helps us to avoid the non-temporal prefetch HW optimizations and
bring this into all cache-levels. It might be worth investigating if
prefetch into L2 will have the same benefit.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
2019-04-17 19:09:25 -07:00

704 lines
20 KiB
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);
}
}