diff options
author | Ming Lei <ming.lei@canonical.com> | 2014-07-06 16:39:26 +0200 |
---|---|---|
committer | Christoph Hellwig <hch@lst.de> | 2014-07-25 17:17:00 -0400 |
commit | 938ece711c5b1ba4fa8e3b9fc8cc03843ae82a5b (patch) | |
tree | b0e463b8a15699a1eb68f92baf3ed52200f255aa /drivers/scsi/virtio_scsi.c | |
parent | 0758f4f732b08b6ef07f2e5f735655cf69fea477 (diff) | |
download | linux-938ece711c5b1ba4fa8e3b9fc8cc03843ae82a5b.tar.bz2 |
virtio-scsi: replace target spinlock with seqcount
The spinlock of tgt_lock is only for serializing read and write
req_vq, one lockless seqcount is enough for the purpose.
On one 16core VM with vhost-scsi backend, the patch can improve
IOPS with 3% on random read test.
Signed-off-by: Ming Lei <ming.lei@canonical.com>
[Add initialization in virtscsi_target_alloc. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Diffstat (limited to 'drivers/scsi/virtio_scsi.c')
-rw-r--r-- | drivers/scsi/virtio_scsi.c | 42 |
1 files changed, 29 insertions, 13 deletions
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c index 308256b5e4cb..cdce502c3c46 100644 --- a/drivers/scsi/virtio_scsi.c +++ b/drivers/scsi/virtio_scsi.c @@ -27,6 +27,7 @@ #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 @@ -75,18 +76,16 @@ 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; @@ -559,19 +558,33 @@ 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 a reader just arrived */ + if (unlikely(atomic_read(&tgt->reqs) > 1)) { + vq = tgt->req_vq; + goto unlock; + } - if (atomic_inc_return(&tgt->reqs) > 1) - vq = tgt->req_vq; - else { 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); - spin_unlock_irqrestore(&tgt->tgt_lock, flags); return vq; } @@ -667,14 +680,17 @@ static int virtscsi_abort(struct scsi_cmnd *sc) static int virtscsi_target_alloc(struct scsi_target *starget) { + struct Scsi_Host *sh = dev_to_shost(starget->dev.parent); + struct virtio_scsi *vscsi = shost_priv(sh); + struct virtio_scsi_target_state *tgt = kmalloc(sizeof(*tgt), GFP_KERNEL); if (!tgt) return -ENOMEM; - spin_lock_init(&tgt->tgt_lock); + seqcount_init(&tgt->tgt_seq); atomic_set(&tgt->reqs, 0); - tgt->req_vq = NULL; + tgt->req_vq = &vscsi->req_vqs[0]; starget->hostdata = tgt; return 0; |