Il 07/05/2014 18:24, Ming Lei ha scritto:
On Tue, 06 May 2014 15:17:15 +0200
Paolo Bonzini <[email protected]> wrote:
Il 06/05/2014 11:26, Ming Lei ha scritto:
Hi Paolo and All,
One question is about ACCESS_ONCE() in virtscsi_pick_vq(),
looks it needn't since both reading and writing tgt->req_vq holds
tgt->tgt_lock.
You're right. It should be possible to avoid the lock in
virtscsi_pick_vq like this:
value = atomic_read(&tgt->reqs);
retry:
if (value != 0) {
old_value = atomic_cmpxchg(&tgt->regs, value, value + 1)
if (old_value != value) {
value = old_value;
goto retry;
}
smp_mb__after_atomic_cmpxchg(); // you get the idea :)
vq = ACCESS_ONCE(tgt->req_vq);
} else {
spin_lock_irqsave(&tgt->tgt_lock, flags);
// tgt->reqs may not be 0 anymore, need to recheck
value = atomic_read(&tgt->reqs);
if (atomic_read(&tgt->reqs) != 0) {
spin_unlock_irqrestore(&tgt->tgt_lock, flags);
goto retry;
}
// tgt->reqs now will remain fixed to 0.
...
tgt->req_vq = vq = ...;
smp_wmb();
atomic_set(&tgt->reqs, 1);
spin_unlock_irqrestore(&tgt->tgt_lock, flags);
}
return vq;
Another approach I thought of is to use percpu spinlock, and
the idea is simple:
- all perpcu locks are held for writing req_vq, and
- only percpu lock is needed for reading req_vq.
What do you think about the below patch?
Per-CPU spinlocks have bad scalability problems, especially if you're
overcommitting. Writing req_vq is not at all rare.
Paolo
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 697fa53..00deab4 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -82,7 +82,7 @@ struct virtio_scsi_vq {
*/
struct virtio_scsi_target_state {
/* This spinlock never held at the same time as vq_lock. */
- spinlock_t tgt_lock;
+ spinlock_t __percpu *lock;
/* Count of outstanding requests. */
atomic_t reqs;
@@ -517,21 +517,46 @@ static struct virtio_scsi_vq *virtscsi_pick_vq(struct
virtio_scsi *vscsi,
{
struct virtio_scsi_vq *vq;
unsigned long flags;
- u32 queue_num;
+ u32 cpu = get_cpu();
+ spinlock_t *lock = per_cpu_ptr(tgt->lock, cpu);
- spin_lock_irqsave(&tgt->tgt_lock, flags);
+ spin_lock_irqsave(lock, flags);
if (atomic_inc_return(&tgt->reqs) > 1)
vq = tgt->req_vq;
else {
- queue_num = smp_processor_id();
+ u32 queue_num = cpu;
+ int i;
+
while (unlikely(queue_num >= vscsi->num_queues))
queue_num -= vscsi->num_queues;
- tgt->req_vq = vq = &vscsi->req_vqs[queue_num];
+ /*
+ * there should be only one writing because of atomic
+ * counter, so we don't worry about deadlock, but
+ * might need to teach lockdep to not complain it
+ */
+ for_each_possible_cpu(i) {
+ spinlock_t *other = per_cpu_ptr(tgt->lock, i);
+ if (i != cpu)
+ spin_lock(other);
+ }
+
+ /* only update req_vq when reqs is one*/
+ if (atomic_read(&tgt->reqs) == 1)
+ tgt->req_vq = vq = &vscsi->req_vqs[queue_num];
+ else
+ vq = tgt->req_vq;
+
+ for_each_possible_cpu(i) {
+ spinlock_t *other = per_cpu_ptr(tgt->lock, i);
+ if (i != cpu)
+ spin_unlock(other);
+ }
}
- spin_unlock_irqrestore(&tgt->tgt_lock, flags);
+ spin_unlock_irqrestore(lock, flags);
+ put_cpu();
return vq;
}
@@ -618,10 +643,22 @@ static int virtscsi_target_alloc(struct scsi_target
*starget)
{
struct virtio_scsi_target_state *tgt =
kmalloc(sizeof(*tgt), GFP_KERNEL);
+ int i;
+
if (!tgt)
return -ENOMEM;
- spin_lock_init(&tgt->tgt_lock);
+ tgt->lock = alloc_percpu(spinlock_t);
+ if (!tgt->lock) {
+ kfree(tgt);
+ return -ENOMEM;
+ }
+
+ for_each_possible_cpu(i) {
+ spinlock_t *lock = per_cpu_ptr(tgt->lock, i);
+ spin_lock_init(lock);
+ }
+
atomic_set(&tgt->reqs, 0);
tgt->req_vq = NULL;
@@ -632,6 +669,7 @@ static int virtscsi_target_alloc(struct scsi_target
*starget)
static void virtscsi_target_destroy(struct scsi_target *starget)
{
struct virtio_scsi_target_state *tgt = starget->hostdata;
+ free_percpu(tgt->lock);
kfree(tgt);
}
Thanks,
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