Il 08/05/2014 12:44, Ming Lei ha scritto:
On Wed, 07 May 2014 18:43:45 +0200
Paolo Bonzini <pbonz...@redhat.com> wrote:
Per-CPU spinlocks have bad scalability problems, especially if you're
overcommitting. Writing req_vq is not at all rare.
OK, thought about it further, and I believe seqcount may
be a match for the case, could you take a look at below patch?
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 13dd500..1adbad7 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -26,6 +26,7 @@
#include <scsi/scsi_host.h>
#include <scsi/scsi_device.h>
#include <scsi/scsi_cmnd.h>
+#include <linux/seqlock.h>
#define VIRTIO_SCSI_MEMPOOL_SZ 64
#define VIRTIO_SCSI_EVENT_LEN 8
@@ -73,18 +74,16 @@ struct virtio_scsi_vq {
* queue, and also lets the driver optimize the IRQ affinity for the virtqueues
* (each virtqueue's affinity is set to the CPU that "owns" the queue).
*
- * tgt_lock is held to serialize reading and writing req_vq. Reading req_vq
- * could be done locklessly, but we do not do it yet.
+ * tgt_seq is held to serialize reading and writing req_vq.
*
* Decrements of reqs are never concurrent with writes of req_vq: before the
* decrement reqs will be != 0; after the decrement the virtqueue completion
* routine will not use the req_vq so it can be changed by a new request.
- * Thus they can happen outside the tgt_lock, provided of course we make reqs
+ * Thus they can happen outside the tgt_seq, provided of course we make reqs
* an atomic_t.
*/
struct virtio_scsi_target_state {
- /* This spinlock never held at the same time as vq_lock. */
- spinlock_t tgt_lock;
+ seqcount_t tgt_seq;
/* Count of outstanding requests. */
atomic_t reqs;
@@ -521,19 +520,33 @@ static struct virtio_scsi_vq *virtscsi_pick_vq(struct
virtio_scsi *vscsi,
unsigned long flags;
u32 queue_num;
- spin_lock_irqsave(&tgt->tgt_lock, flags);
+ local_irq_save(flags);
+ if (atomic_inc_return(&tgt->reqs) > 1) {
+ unsigned long seq;
+
+ do {
+ seq = read_seqcount_begin(&tgt->tgt_seq);
+ vq = tgt->req_vq;
+ } while (read_seqcount_retry(&tgt->tgt_seq, seq));
+ } else {
+ /* no writes can be concurrent because of atomic_t */
+ write_seqcount_begin(&tgt->tgt_seq);
+
+ /* keep previous req_vq if there is reader found */
+ if (unlikely(atomic_read(&tgt->reqs) > 1)) {
+ vq = tgt->req_vq;
+ goto unlock;
+ }
queue_num = smp_processor_id();
while (unlikely(queue_num >= vscsi->num_queues))
queue_num -= vscsi->num_queues;
tgt->req_vq = vq = &vscsi->req_vqs[queue_num];
+ unlock:
+ write_seqcount_end(&tgt->tgt_seq);
}
+ local_irq_restore(flags);
I find this harder to think about than the double-check with a
spin_lock_irqsave in the middle, and the read side is not lock free anymore.
Paolo
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